diff --git a/.github/workflows/type-check.yml b/.github/workflows/type-check.yml
new file mode 100644
index 0000000..9e3b21e
--- /dev/null
+++ b/.github/workflows/type-check.yml
@@ -0,0 +1,23 @@
+name: Type Check
+
+on:
+  pull_request:
+    branches:
+      - main
+jobs:
+  typecheck:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '24'
+
+      - name: Install dependencies
+        run: npm i
+
+      - name: Run type check
+        run: npm run check:types
\ No newline at end of file
diff --git a/lib/compile/csdl2openapi.js b/lib/compile/csdl2openapi.js
index d09d545..50518f0 100644
--- a/lib/compile/csdl2openapi.js
+++ b/lib/compile/csdl2openapi.js
@@ -3,7 +3,7 @@
 */
 const cds = require('@sap/cds');
 var pluralize = require('pluralize')
-const DEBUG = cds.debug('openapi');  // Initialize cds.debug with the 'openapi' 
+const DEBUG = cds.debug('openapi');  // Initialize cds.debug with the 'openapi'
 
 
 //TODO
@@ -104,7 +104,8 @@ module.exports.csdl2openapi = function (
     const namespace = { 'Edm': 'Edm' };
     const namespaceUrl = {};
     const voc = {};
-    const requiredSchemas = { list: [], used: {} };
+    /** @type {{ list: { namespace:string, name: string, suffix: string }[], used: Record<string, boolean> }} */
+    const requiredSchemas = { list:  [], used: {} };
 
     preProcess(csdl, boundOverloads, derivedTypes, alias, namespace, namespaceUrl, voc);
 
@@ -157,7 +158,7 @@ module.exports.csdl2openapi = function (
 
         for (const [key, value] of Object.entries(containerSchema)) {
             if (key.startsWith('@OpenAPI.Extensions')) {
-                const annotationProperties = key.split('@OpenAPI.Extensions.')[1];
+                const annotationProperties = key.split('@OpenAPI.Extensions.')[1] ?? ''
                 const keys = annotationProperties.split('.');
                 if (!keys[0].startsWith("x-sap-")) {
                     keys[0] = (keys[0].startsWith("sap-") ? "x-" : "x-sap-") + keys[0];
@@ -171,25 +172,25 @@ module.exports.csdl2openapi = function (
         }
         let extensionEnums = {
             "x-sap-compliance-level": {allowedValues: ["sap:base:v1", "sap:core:v1", "sap:core:v2" ] } ,
-            "x-sap-api-type": {allowedValues: [ "ODATA", "ODATAV4", "REST" , "SOAP"] }, 
+            "x-sap-api-type": {allowedValues: [ "ODATA", "ODATAV4", "REST" , "SOAP"] },
             "x-sap-direction": {allowedValues: ["inbound", "outbound", "mixed"] , default : "inbound" },
             "x-sap-dpp-entity-semantics": {allowedValues: ["sap:DataSubject", "sap:DataSubjectDetails", "sap:Other"] },
             "x-sap-dpp-field-semantics": {allowedValues: ["sap:DataSubjectID", "sap:ConsentID", "sap:PurposeID", "sap:ContractRelatedID", "sap:LegalEntityID", "sap:DataControllerID", "sap:UserID", "sap:EndOfBusinessDate", "sap:BlockingDate", "sap:EndOfRetentionDate"] },
         };
-        checkForExtentionEnums(extensionObj, extensionEnums);
+        checkForExtensionEnums(extensionObj, extensionEnums);
 
-        let extenstionSchema = {    
+        let extensionSchema = {
             "x-sap-stateInfo": ['state', 'deprecationDate', 'decomissionedDate', 'link'],
             "x-sap-ext-overview": ['name', 'values'],
             "x-sap-deprecated-operation" : ['deprecationDate', 'successorOperationRef', "successorOperationId"],
             "x-sap-odm-semantic-key" : ['name', 'values'],
         };
 
-        checkForExtentionSchema(extensionObj, extenstionSchema);
+        checkForExtentionSchema(extensionObj, extensionSchema);
         return extensionObj;
     }
 
-    function checkForExtentionEnums(extensionObj, extensionEnums){
+    function checkForExtensionEnums(extensionObj, extensionEnums){
         for (const [key, value] of Object.entries(extensionObj)) {
             if(extensionEnums[key] && extensionEnums[key].allowedValues && !extensionEnums[key].allowedValues.includes(value)){
                 if(extensionEnums[key].default){
@@ -202,14 +203,14 @@ module.exports.csdl2openapi = function (
         }
     }
 
-    function checkForExtentionSchema(extensionObj, extenstionSchema) {
+    function checkForExtentionSchema(extensionObj, extensionSchema) {
         for (const [key, value] of Object.entries(extensionObj)) {
-            if (extenstionSchema[key]) {
+            if (extensionSchema[key]) {
                 if (Array.isArray(value)) {
-                    extensionObj[key] = value.filter((v) => extenstionSchema[key].includes(v));
+                    extensionObj[key] = value.filter((v) => extensionSchema[key].includes(v));
                 } else if (typeof value === "object" && value !== null) {
                     for (const field in value) {
-                        if (!extenstionSchema[key].includes(field)) {
+                        if (!extensionSchema[key].includes(field)) {
                             delete extensionObj[key][field];
                         }
                     }
@@ -223,9 +224,9 @@ module.exports.csdl2openapi = function (
         if (resObj[openapiProperty] === undefined) {
             resObj[openapiProperty] = {};
         }
-    
+
         let node = resObj[openapiProperty];
-    
+
         // traverse the annotation property and define the objects if they're not defined
         for (let nestedIndex = 1; nestedIndex < keys.length - 1; nestedIndex++) {
             const nestedElement = keys[nestedIndex];
@@ -234,14 +235,15 @@ module.exports.csdl2openapi = function (
             }
             node = node[nestedElement];
         }
-    
+
         // set value annotation property
         node[keys[keys.length - 1]] = value;
     }
-        
+
     if (!csdl.$EntityContainer) {
-        delete openapi.servers;
-        delete openapi.tags;
+        // explicit cast required as .servers and .tags are not declared as optional
+        delete /**@type{any}*/(openapi).servers;
+        delete /**@type{any}*/(openapi).tags;
     }
 
     security(openapi, entityContainer);
@@ -301,13 +303,14 @@ module.exports.csdl2openapi = function (
             Object.keys(schema.$Annotations || {}).forEach(target => {
                 const annotations = schema.$Annotations[target];
                 const segments = target.split('/');
-                const open = segments[0].indexOf('(');
+                const firstSegment = segments[0];
+                const open = firstSegment.indexOf('(');
                 let element;
                 if (open == -1) {
-                    element = modelElement(segments[0]);
+                    element = modelElement(firstSegment);
                 } else {
-                    element = modelElement(segments[0].substring(0, open));
-                    let args = segments[0].substring(open + 1, segments[0].length - 1);
+                    element = modelElement(firstSegment.substring(0, open));
+                    let args = firstSegment.substring(open + 1, firstSegment.length - 1);
                     element = element.find(
                         (overload) =>
                             (overload.$Kind == "Action" &&
@@ -337,23 +340,21 @@ module.exports.csdl2openapi = function (
                         case 1:
                             Object.assign(element, annotations);
                             break;
-                        case 2:
+                        case 2: {
+                            const secondSegment = /**@type{string}*/(segments[1])
                             if (['Action', 'Function'].includes(element.$Kind)) {
-                                if (segments[1] == '$ReturnType') {
+                                if (secondSegment === '$ReturnType') {
                                     if (element.$ReturnType)
                                         Object.assign(element.$ReturnType, annotations);
                                 } else {
-                                    const parameter = element.$Parameter.find(p => p.$Name == segments[1]);
+                                    const parameter = element.$Parameter.find(p => p.$Name == secondSegment);
                                     Object.assign(parameter, annotations);
                                 }
-                            } else {
-                                if (element[segments[1]]) {
-                                    Object.assign(element[segments[1]], annotations);
-                                } else {
-                                    // DEBUG?.(`Invalid annotation target '${target}'`)
-                                }
+                            } else if (element[secondSegment]) {
+                                Object.assign(element[secondSegment], annotations);
                             }
                             break;
+                        }
                         default:
                             DEBUG?.('More than two annotation target path segments');
                     }
@@ -673,8 +674,8 @@ module.exports.csdl2openapi = function (
      * @param {object} root Root model element
      * @param {string} sourceName Name of path source
      * @param {string} targetName Name of path target
-     * @param {string} target Target container child of path
-     * @param {integer} level Number of navigation segments so far
+     * @param {null | import('./types').TargetRestrictions[]} target Target container child of path
+     * @param {number} level Number of navigation segments so far
      * @param {string} navigationPath Path for finding navigation restrictions
      */
     function pathItems(paths, prefix, prefixParameters, element, root, sourceName, targetName, target, level, navigationPath) {
@@ -753,8 +754,8 @@ module.exports.csdl2openapi = function (
      * @param {object} root Root model element
      * @param {string} sourceName Name of path source
      * @param {string} targetName Name of path target
-     * @param {string} target Target container child of path
-     * @param {integer} level Number of navigation segments so far
+     * @param {null | object} target Target container child of path
+     * @param {number} level Number of navigation segments so far
      * @param {string} navigationPath Path for finding navigation restrictions
      * @param {object} restrictions Navigation property restrictions of navigation segment
      * @param {array} nonExpandable Non-expandable navigation properties
@@ -792,13 +793,13 @@ module.exports.csdl2openapi = function (
      * @param {string} name Name of navigation segment
      * @param {string} sourceName Name of path source
      * @param {string} targetName Name of path target
-     * @param {string} target Target container child of path
-     * @param {integer} level Number of navigation segments so far
+     * @param {null | import('./types').TargetRestrictions[]} target Target container child of path
+     * @param {number} level Number of navigation segments so far
      * @param {object} restrictions Navigation property restrictions of navigation segment
      */
     function operationCreate(pathItem, element, name, sourceName, targetName, target, level, restrictions) {
-        const insertRestrictions = restrictions.InsertRestrictions || target && target[voc.Capabilities.InsertRestrictions] || {};
-        let countRestrictions = target && (target[voc.Capabilities.CountRestrictions]?.Countable === false); // count property will be added if CountRestrictions is false
+        const insertRestrictions = restrictions.InsertRestrictions || target?.[voc.Capabilities.InsertRestrictions] || {};
+        let countRestrictions = target?.[voc.Capabilities.CountRestrictions]?.Countable === false // count property will be added if CountRestrictions is false
         if (insertRestrictions.Insertable !== false) {
             const lname = pluralize.singular(splitName(name));
             const type = modelElement(element.$Type);
@@ -836,7 +837,7 @@ module.exports.csdl2openapi = function (
      * @param {string} operation Operation (verb)
      * @param {string} name Name of navigation segment
      * @param {string} sourceName Name of path source
-     * @param {integer} level Number of navigation segments so far
+     * @param {number} level Number of navigation segments so far
      * @param {boolean} collection Access a collection
      * @param {boolean} byKey Access by key
      * @return {string} Operation Text
@@ -860,8 +861,8 @@ module.exports.csdl2openapi = function (
      * @param {string} name Name of navigation segment
      * @param {string} sourceName Name of path source
      * @param {string} targetName Name of path target
-     * @param {string} target Target container child of path
-     * @param {integer} level Number of navigation segments so far
+     * @param {null |import('./types').TargetRestrictions[]} target Target container child of path
+     * @param {number} level Number of navigation segments so far
      * @param {object} restrictions Navigation property restrictions of navigation segment
      * @param {boolean} byKey Read by key
      * @param {array} nonExpandable Non-expandable navigation properties
@@ -901,9 +902,12 @@ module.exports.csdl2openapi = function (
             customParameters(operation, byKey ? readByKeyRestrictions || readRestrictions : readRestrictions);
 
             if (collection) {
+                // @ts-expect-error - see FIXME in optionTop and optionSkip
                 optionTop(operation.parameters, target, restrictions);
+                // @ts-expect-error
                 optionSkip(operation.parameters, target, restrictions);
                 if (csdl.$Version >= '4.0') optionSearch(operation.parameters, target, restrictions);
+                // @ts-expect-error
                 optionFilter(operation.parameters, target, restrictions);
                 optionCount(operation.parameters, target);
                 optionOrderBy(operation.parameters, element, target, restrictions);
@@ -960,10 +964,10 @@ module.exports.csdl2openapi = function (
     /**
      * Add parameter for query option $count
      * @param {Array} parameters Array of parameters to augment
-     * @param {string} target Target container child of path
+     * @param {null | import('./types').TargetRestrictions[]} target Target container child of path
      */
     function optionCount(parameters, target) {
-        const targetRestrictions = target && target[voc.Capabilities.CountRestrictions];
+        const targetRestrictions = target?.[voc.Capabilities.CountRestrictions];
         const targetCountable = target == null
             || targetRestrictions == null
             || targetRestrictions.Countable !== false;
@@ -980,11 +984,11 @@ module.exports.csdl2openapi = function (
      * Add parameter for query option $expand
      * @param {Array} parameters Array of parameters to augment
      * @param {object} element Model element of navigation segment
-     * @param {string} target Target container child of path
+     * @param {null | import('./types').TargetRestrictions[]} target Target container child of path
      * @param {array} nonExpandable Non-expandable navigation properties
      */
     function optionExpand(parameters, element, target, nonExpandable) {
-        const targetRestrictions = target && target[voc.Capabilities.ExpandRestrictions];
+        const targetRestrictions = target?.[voc.Capabilities.ExpandRestrictions];
         const supported = targetRestrictions == null || targetRestrictions.Expandable != false;
         if (supported) {
             const expandItems = ['*'].concat(navigationPaths(element).filter(path => !nonExpandable.includes(path)));
@@ -1015,7 +1019,7 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
      * Collect navigation paths of a navigation segment and its potentially structured components
      * @param {object} element Model element of navigation segment
      * @param {string} prefix Navigation prefix
-     * @param {integer} level Number of navigation segments so far
+     * @param {number} level Number of navigation segments so far
      * @return {Array} Array of navigation property paths
      */
     function navigationPaths(element, prefix = '', level = 0) {
@@ -1035,11 +1039,11 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
     /**
      * Add parameter for query option $filter
      * @param {Array} parameters Array of parameters to augment
-     * @param {string} target Target container child of path
+     * @param {null | import('./types').TargetRestrictions} target Target container child of path
      * @param {object} restrictions Navigation property restrictions of navigation segment
      */
     function optionFilter(parameters, target, restrictions) {
-        const filterRestrictions = restrictions.FilterRestrictions || target && target[voc.Capabilities.FilterRestrictions] || {};
+        const filterRestrictions = restrictions.FilterRestrictions || target?.[voc.Capabilities.FilterRestrictions] || {};
 
         if (filterRestrictions.Filterable !== false) {
             const filter = {
@@ -1067,11 +1071,11 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
      * Add parameter for query option $orderby
      * @param {Array} parameters Array of parameters to augment
      * @param {object} element Model element of navigation segment
-     * @param {string} target Target container child of path
+     * @param {null | import('./types').TargetRestrictions[]} target Target container child of path
      * @param {object} restrictions Navigation property restrictions of navigation segment
      */
     function optionOrderBy(parameters, element, target, restrictions) {
-        const sortRestrictions = restrictions.SortRestrictions || target && target[voc.Capabilities.SortRestrictions] || {};
+        const sortRestrictions = restrictions.SortRestrictions || target?.[voc.Capabilities.SortRestrictions] || {};
 
         if (sortRestrictions.Sortable !== false) {
             const nonSortable = {};
@@ -1105,7 +1109,7 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
 
     /**
      * Unpack EnumMember value if it uses CSDL JSON CS01 style, like CAP does
-     * @param {string or object} path Qualified name of referenced type
+     * @param {string | object} member Qualified name of referenced type
      * @return {object} Reference Object
      */
     function enumMember(member) {
@@ -1117,7 +1121,7 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
 
     /**
      * Unpack NavigationPropertyPath value if it uses CSDL JSON CS01 style, like CAP does
-     * @param {string or object} path Qualified name of referenced type
+     * @param {string | object} path Qualified name of referenced type
      * @return {object} Reference Object
      */
     function navigationPropertyPath(path) {
@@ -1129,7 +1133,7 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
 
     /**
      * Unpack PropertyPath value if it uses CSDL JSON CS01 style, like CAP does
-     * @param {string or object} path Qualified name of referenced type
+     * @param {string | object} path Qualified name of referenced type
      * @return {object} Reference Object
      */
     function propertyPath(path) {
@@ -1221,11 +1225,11 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
     /**
      * Add parameter for query option $search
      * @param {Array} parameters Array of parameters to augment
-     * @param {string} target Target container child of path
+     * @param {null | import('./types').TargetRestrictions[]} target Target container child of path
      * @param {object} restrictions Navigation property restrictions of navigation segment
      */
     function optionSearch(parameters, target, restrictions) {
-        const searchRestrictions = restrictions.SearchRestrictions || target && target[voc.Capabilities.SearchRestrictions] || {};
+        const searchRestrictions = restrictions.SearchRestrictions ?? target?.[voc.Capabilities.SearchRestrictions] ?? {};
 
         if (searchRestrictions.Searchable !== false) {
             if (searchRestrictions[voc.Core.Description]) {
@@ -1245,11 +1249,11 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
      * Add parameter for query option $select
      * @param {Array} parameters Array of parameters to augment
      * @param {object} element Model element of navigation segment
-     * @param {string} target Target container child of path
+     * @param {null | import('./types').TargetRestrictions[]} target Target container child of path
      * @param {object} restrictions Navigation property restrictions of navigation segment
      */
     function optionSelect(parameters, element, target, restrictions) {
-        const selectSupport = restrictions.SelectSupport || target && target[voc.Capabilities.SelectSupport] || {};
+        const selectSupport = restrictions.SelectSupport ?? target?.[voc.Capabilities.SelectSupport] ?? {};
 
         if (selectSupport.Supported !== false) {
             const type = modelElement(element.$Type) || {};
@@ -1280,7 +1284,7 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
     /**
      * Add parameter for query option $skip
      * @param {Array} parameters Array of parameters to augment
-     * @param {string} target Target container child of path
+     * @param {Record<string, boolean>} target Target container child of path  FIXME: this seems to be an incorrect use of TargetRestrictions
      * @param {object} restrictions Navigation property restrictions of navigation segment
      */
     function optionSkip(parameters, target, restrictions) {
@@ -1298,13 +1302,13 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
     /**
      * Add parameter for query option $top
      * @param {Array} parameters Array of parameters to augment
-     * @param {string} target Target container child of path
+     * @param {Record<string, boolean>} target Target container child of path  FIXME: this seems to be an incorrect use of TargetRestrictions
      * @param {object} restrictions Navigation property restrictions of navigation segment
      */
     function optionTop(parameters, target, restrictions) {
         const supported = restrictions.TopSupported !== undefined
             ? restrictions.TopSupported
-            : target == null || target[voc.Capabilities.TopSupported] !== false;
+            : target == null || target?.[voc.Capabilities.TopSupported] !== false;
 
         if (supported) {
             parameters.push({
@@ -1319,14 +1323,14 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
      * @param {object} element Model element of navigation segment
      * @param {string} name Name of navigation segment
      * @param {string} sourceName Name of path source
-     * @param {string} target Target container child of path
-     * @param {integer} level Number of navigation segments so far
+     * @param {null | import('./types').TargetRestrictions[]} target Target container child of path
+     * @param {number} level Number of navigation segments so far
      * @param {object} restrictions Navigation property restrictions of navigation segment
      * @param {boolean} byKey Update by key
      */
-    function operationUpdate(pathItem, element, name, sourceName, target, level, restrictions, byKey) {
-        const updateRestrictions = restrictions.UpdateRestrictions || target && target[voc.Capabilities.UpdateRestrictions] || {};
-        let countRestrictions = target && (target[voc.Capabilities.CountRestrictions]?.Countable === false);
+    function operationUpdate(pathItem, element, name, sourceName, target, level, restrictions, byKey = false) {
+        const updateRestrictions = restrictions.UpdateRestrictions || target?.[voc.Capabilities.UpdateRestrictions] || {};
+        let countRestrictions = target?.[voc.Capabilities.CountRestrictions]?.Countable === false;
         if (updateRestrictions.Updatable !== false) {
             const type = modelElement(element.$Type);
             const operation = {
@@ -1356,14 +1360,14 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
      * @param {object} element Model element of navigation segment
      * @param {string} name Name of navigation segment
      * @param {string} sourceName Name of path source
-     * @param {string} target Target container child of path
-     * @param {integer} level Number of navigation segments so far
+     * @param {null | import('./types').TargetRestrictions[]} target Target container child of path
+     * @param {number} level Number of navigation segments so far
      * @param {object} restrictions Navigation property restrictions of navigation segment
      * @param {boolean} byKey Delete by key
      */
-    function operationDelete(pathItem, element, name, sourceName, target, level, restrictions, byKey) {
-        const deleteRestrictions = restrictions.DeleteRestrictions || target && target[voc.Capabilities.DeleteRestrictions] || {};
-        let countRestrictions = target && (target[voc.Capabilities.CountRestrictions]?.Countable === false);
+    function operationDelete(pathItem, element, name, sourceName, target, level, restrictions, byKey = false) {
+        const deleteRestrictions = restrictions.DeleteRestrictions || target?.[voc.Capabilities.DeleteRestrictions] || {};
+        let countRestrictions = target?.[voc.Capabilities.CountRestrictions]?.Countable === false
         if (deleteRestrictions.Deletable !== false) {
             pathItem.delete = {
                 summary: deleteRestrictions.Description || operationSummary('Deletes', name, sourceName, level, element.$Collection, byKey),
@@ -1382,11 +1386,11 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
      * @param {Array} prefixParameters Parameter Objects for prefix
      * @param {object} type Entity type object of navigation segment
      * @param {string} sourceName Name of path source
-     * @param {integer} level Number of navigation segments so far
+     * @param {number} level Number of navigation segments so far
      * @param {string} navigationPrefix Path for finding navigation restrictions
      */
     function pathItemsWithNavigation(paths, prefix, prefixParameters, type, root, sourceName, level, navigationPrefix) {
-        const navigationRestrictions = root[voc.Capabilities.NavigationRestrictions] || {};
+        const navigationRestrictions = root[voc.Capabilities.NavigationRestrictions] ?? {};
         const rootNavigable = level == 0 && enumMember(navigationRestrictions.Navigability) != 'None'
             || level == 1 && enumMember(navigationRestrictions.Navigability) != 'Single'
             || level > 1;
@@ -1416,7 +1420,7 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
      * @param {object} type Structured type
      * @param {object} map Map of navigation property paths and their types
      * @param {string} prefix Navigation prefix
-     * @param {integer} level Number of navigation segments so far
+     * @param {number} level Number of navigation segments so far
      * @return {object} Map of navigation property paths and their types
      */
     function navigationPathMap(type, map = {}, prefix = '', level = 0) {
@@ -1467,7 +1471,7 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
     /**
      * Key for path item
      * @param {object} entityType Entity Type object
-     * @param {integer} level Number of navigation segments so far
+     * @param {number} level Number of navigation segments so far
      * @return {object} key: Key segment, parameters: key parameters
      */
     function entityKey(entityType, level) {
@@ -1516,7 +1520,7 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
 
     /**
       * Prefix for key value in key segment
-      * @param {typename} Qualified name of key property type
+      * @param {string} typename Qualified name of key property type
       * @return {string} value prefix
       */
     function pathValuePrefix(typename) {
@@ -1529,7 +1533,7 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
 
     /**
      * Suffix for key value in key segment
-     * @param {typename} Qualified name of key property type
+     * @param {string} typename Qualified name of key property type
      * @return {string} value prefix
      */
     function pathValueSuffix(typename) {
@@ -1582,7 +1586,7 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
      * @param {string} actionName Qualified name of function
      * @param {object} overload Function overload
      * @param {string} sourceName Name of path source
-     * @param {string} actionImport Action import
+     * @param {object} actionImport Action import
      */
     function pathItemAction(paths, prefix, prefixParameters, actionName, overload, sourceName, actionImport = {}) {
         const name = actionName.indexOf('.') === -1 ? actionName : nameParts(actionName).name;
@@ -1793,7 +1797,7 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
 
     /**
      * Construct Responses Object
-     * @param {string} code HTTP response code
+     * @param {string | number} code HTTP response code
      * @param {string} description Description
      * @param {object} type Response type object
      * @param {array} errors Array of operation-specific status codes with descriptions
@@ -2174,7 +2178,6 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
 
     /**
      * Construct OData error response
-     * @return {object} Error response schema
      */
     function error() {
         const err = {
@@ -2218,8 +2221,9 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
                 },
                 required: ['lang', 'value']
             };
-            delete err.properties.error.properties.details;
-            delete err.properties.error.properties.target;
+            // explicit cast required, as .details and .target are no inferred as optional above
+            delete /**@type{any}*/(err.properties.error.properties).details;
+            delete /**@type{any}*/(err.properties.error.properties).target;
         }
 
         return err;
@@ -2227,7 +2231,6 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
 
     /**
      * Construct OData count response
-     * @return {object} Count response schema
      */
     function count() {
         return {
@@ -2241,17 +2244,19 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
 
     /**
      * Construct Schema Object for model object referencing a type
-     * @param {object} modelElement referencing a type
+     * @param {object} element referencing a type
      * @return {object} Schema Object
      */
     function getSchema(element, suffix = '', forParameter = false, forFunction = false) {
+
+        /** @type {import('./types').Schema} */
         let s = {};
         switch (element.$Type) {
             case 'Edm.AnnotationPath':
             case 'Edm.ModelElementPath':
             case 'Edm.NavigationPropertyPath':
             case 'Edm.PropertyPath':
-                s.type = 'string';
+                s = { type: 'string' };
                 break;
             case 'Edm.Binary':
                 s = {
@@ -2261,7 +2266,7 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
                 if (element.$MaxLength) s.maxLength = Math.ceil(4 * element.$MaxLength / 3);
                 break;
             case 'Edm.Boolean':
-                s.type = 'boolean';
+                s = { type: 'boolean' };
                 break;
             case 'Edm.Byte':
                 s = {
@@ -2284,9 +2289,10 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
                     example: '2017-04-13T15:51:04' + (isNaN(element.$Precision) || element.$Precision === 0 ? '' : '.' + '0'.repeat(element.$Precision)) + 'Z'
                 };
                 break;
-            case 'Edm.Decimal':
+            case 'Edm.Decimal': {
+                const preDecimal = /** @type {const}*/({ type: 'number', format: 'decimal' })
                 s = {
-                    anyOf: [{ type: 'number', format: 'decimal' }, { type: 'string' }],
+                    anyOf: [preDecimal, { type: 'string' }],
                     example: 0
                 };
                 if (!isNaN(element.$Precision)) s['x-sap-precision'] = element.$Precision;
@@ -2294,19 +2300,20 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
                 // eslint-disable-next-line no-case-declarations
                 let scale = !isNaN(element.$Scale) ? element.$Scale : null;
                 if (scale !== null) {
-                    // Node.js 12.13.0 has problems with negative exponents, 10 ** -5 --> 0.000009999999999999999
+                    // Node.js 20 has problems with negative exponents, 10 ** -5 --> 0.000009999999999999999
                     if (scale <= 0)
-                        s.anyOf[0].multipleOf = 10 ** -scale;
+                        preDecimal.multipleOf = 10 ** -scale;
                     else
-                        s.anyOf[0].multipleOf = 1 / 10 ** scale;
+                        preDecimal.multipleOf = 1 / 10 ** scale;
                 }
                 if (element.$Precision < 16) {
                     let limit = 10 ** (element.$Precision - scale);
                     let delta = 10 ** -scale;
-                    s.anyOf[0].maximum = limit - delta;
-                    s.anyOf[0].minimum = -s.anyOf[0].maximum;
+                    preDecimal.maximum = limit - delta;
+                    preDecimal.minimum = -preDecimal.maximum;
                 }
                 break;
+            }
             case 'Edm.Double':
                 s = {
                     anyOf: [{ type: 'number', format: 'double' }, { type: 'string' }],
@@ -2383,11 +2390,13 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
                 }
                 break;
             case 'Edm.String':
-            case undefined:
-                s.type = 'string';
+            case undefined: {
+                s = { type: 'string' };
                 if (element.$MaxLength) s.maxLength = element.$MaxLength;
-                getPattern(s, element);
+                const pattern = element[voc.Validation.Pattern];
+                if (pattern) s.pattern = pattern;
                 break;
+            }
             case 'Edm.TimeOfDay':
                 s = {
                     type: 'string',
@@ -2428,41 +2437,55 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
             s.example = element[voc.Core.Example].Value;
         }
 
+        /** @returns {s is import('./types.d.ts').StringSchema} */
+        const isStringSchema = s => s?.type === 'string'
+        /** @returns {s is import('./types.d.ts').NumberSchema} */
+        const isNumberSchema = s => s?.type === 'number' || s?.type === 'integer'
+        /** @returns {s is import('./types.d.ts').AnyOf} */
+        const isAnyOfSchema = s => Boolean(s?.anyOf)
+
         if (forFunction) {
             if (s.example && typeof s.example === "string") {
                 s.example = `${pathValuePrefix(element.$Type)}${s.example
                     }${pathValueSuffix(element.$Type)} `;
             }
-            if (s.pattern) {
-                const pre = pathValuePrefix(element.$Type);
-                const suf = pathValueSuffix(element.$Type);
-                s.pattern = s.pattern.replace(/^\^/, `^ ${pre} (`);
-                s.pattern = s.pattern.replace(/\$$/, `)${suf} $`);
-            } else if (!element.$Type || element.$Type === "Edm.String") {
-                s.pattern = "^'([^']|'')*'$";
+            if (isStringSchema(s)) {
+                if (s.pattern) {
+                    const pre = pathValuePrefix(element.$Type);
+                    const suf = pathValueSuffix(element.$Type);
+                    s.pattern = s.pattern.replace(/^\^/, `^ ${pre} (`);
+                    s.pattern = s.pattern.replace(/\$$/, `)${suf} $`);
+                } else if (!element.$Type || element.$Type === "Edm.String") {
+                    s.pattern = "^'([^']|'')*'$";
+                }
             }
             if (element.$Nullable) {
                 s.default = "null";
-                if (s.pattern) {
-                    s.pattern = s.pattern.replace(/^\^/, "^(null|");
-                    s.pattern = s.pattern.replace(/\$$/, ")$");
+                if (isStringSchema(s) && s.pattern) {
+                    s.pattern = s.pattern
+                        .replace(/^\^/, "^(null|")
+                        .replace(/\$$/, ")$");
                 }
             }
         }
 
         if (element[voc.Validation.Maximum] != undefined) {
             if (s.$ref) s = { allOf: [s] };
-            if (s.anyOf) {
+            if (isAnyOfSchema(s) && isNumberSchema(s.anyOf[0])) {
                 s.anyOf[0].maximum = element[voc.Validation.Maximum];
             }
+            // TODO: this implies that we could be handling an AnyOfSchema here. So exclusiveMinimum is attach to that? Or to its first element, as above?
+            // @ts-expect-error
             if (element[voc.Validation.Maximum + voc.Validation.Exclusive]) s.exclusiveMaximum = true;
         }
 
         if (element[voc.Validation.Minimum] != undefined) {
             if (s.$ref) s = { allOf: [s] };
-            if (s.anyOf) {
+            if (isAnyOfSchema(s) && isNumberSchema(s.anyOf[0])) {
                 s.anyOf[0].minimum = element[voc.Validation.Minimum];
             }
+            // TODO: see above
+            // @ts-expect-error
             if (element[voc.Validation.Minimum + voc.Validation.Exclusive]) s.exclusiveMinimum = true;
         }
 
@@ -2500,16 +2523,6 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
         if (values) schema.enum = values.map(record => record.Value);
     }
 
-    /**
-     * Add pattern to Schema Object for string-like model element
-     * @param {object} schema Schema Object to augment
-     * @param {object} element Model element
-     */
-    function getPattern(schema, element) {
-        const pattern = element[voc.Validation.Pattern];
-        if (pattern) schema.pattern = pattern;
-    }
-
     /**
      * Construct Reference Object for a type
      * @param {string} typename Qualified name of referenced type
@@ -2664,5 +2677,4 @@ see [Expand](http://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part1-prot
     function isIdentifier(name) {
         return !name.startsWith('$') && !name.includes('@');
     }
-    
 };
diff --git a/lib/compile/index.js b/lib/compile/index.js
index 9c522b3..4f6cdf5 100644
--- a/lib/compile/index.js
+++ b/lib/compile/index.js
@@ -184,4 +184,5 @@ function _servicePath(csdl, csn, protocols) {
 
     return paths;
   }
+  return {}
 }
\ No newline at end of file
diff --git a/lib/compile/types.d.ts b/lib/compile/types.d.ts
new file mode 100644
index 0000000..1a8f7b9
--- /dev/null
+++ b/lib/compile/types.d.ts
@@ -0,0 +1,50 @@
+type StringSchema = {
+    type: 'string'
+    format?: 'base64url' | 'uuid' | 'time' | 'date' | 'date-time' | 'duration'
+    maxLength?: number
+    example?: string
+    pattern?: string
+}
+
+type NumberSchema = {
+    type: 'number' | 'integer'
+    format?: 'float' | 'double' | 'decimal' | 'uint8' | 'int8' | 'int16' | 'int32' | 'int64'
+    multipleOf?: number
+    example?: number,
+    minimum?: number
+    maximum?: number
+    exclusiveMinimum?: boolean
+    exclusiveMaximum?: boolean
+}
+
+type BooleanSchema = {
+    type: 'boolean'
+}
+
+type ArraySchema = {
+    type: 'array',
+    items: Schema
+}
+
+type Meta = {
+    nullable?: boolean
+    default?: unknown
+    example?: string | number,
+    description?: string
+    '$ref'?: unknown
+}
+
+type SingleSchema = (StringSchema | NumberSchema | BooleanSchema | ArraySchema) & Meta
+
+type AnyOf = { anyOf: Schema[] } & Meta
+type AllOf = { allOf: Schema[] } & Meta
+type MultiSchema = AnyOf | AllOf
+
+export type Schema = (SingleSchema | MultiSchema)
+
+
+
+export type TargetRestrictions = {
+    Countable?: boolean
+    Expandable?: boolean
+}
\ No newline at end of file
diff --git a/package.json b/package.json
index 327d5be..a7a2bf8 100644
--- a/package.json
+++ b/package.json
@@ -22,7 +22,8 @@
   ],
   "scripts": {
     "test": "npx jest --silent",
-    "lint": "npx eslint ."
+    "lint": "npx eslint .",
+    "check:types": "npx tsc --noEmit"
   },
   "dependencies": {
     "pluralize": "^8.0.0"
@@ -31,7 +32,9 @@
     "@sap/cds": ">=7.6"
   },
   "devDependencies": {
+    "@types/node": "^24.3.0",
+    "eslint": "^8.56.0",
     "jest": ">=29",
-    "eslint": "^8.56.0"
+    "typescript": "^5.9.2"
   }
 }
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..c14df73
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,31 @@
+{
+  // Visit https://aka.ms/tsconfig to read more about this file
+  "compilerOptions": {
+    "module": "nodenext",
+    "target": "esnext",
+    "lib": ["esnext"],
+    "types": ["node"],
+    "allowJs": true,
+    "checkJs": true,
+    "noUncheckedIndexedAccess": false,
+    "exactOptionalPropertyTypes": true,
+    "noImplicitOverride": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "noFallthroughCasesInSwitch": true,
+    "noPropertyAccessFromIndexSignature": true,
+    "strict": true,
+    "jsx": "react-jsx",
+    "verbatimModuleSyntax": true,
+    "isolatedModules": true,
+    "noUncheckedSideEffectImports": true,
+    "moduleDetection": "force",
+    "skipLibCheck": true,
+    // enable after first round
+    "noImplicitReturns": false,
+    "noImplicitAny": false,
+  },
+  "exclude": [
+    "**/*.test.js"
+  ]
+}