Support language N4JS (#15089)

* n4js initial commit

* incorporate feedback from user

* add tests

* fix media type in case of DELETE method

* fix media type

* some minor fixes

* options fix for booleans

* small fixes

* generated files by ./bin/utils/ensure-up-to-date

* remove String::toLowerCase due to de.thetaphi:forbiddenapis

* adjust test expectation

* fix test expectations

* fix test expectation

* add note to section 'Languages/Generators'

* remove file according to review

samples/client/petstore/n4js/api/ApiHelper.n4js

@@ -0,0 +1,113 @@
+
+/**
+ * Implemented by client
+ */
+export public interface ~ApiExecuterI {
+    public <R, E> async exec(
+        method: string,
+        path: string,
+        pathParams: ~Object+,
+        queryParams: ~Object+,
+        headerParams: ~Object+,
+        payloadContentType: string,
+        body: any+) : Promise<R, Object|ApiError<E>>;
+}
+
+export public interface ~ApiError<T> {
+    public resultBody?: T;
+}
+
+export public function checkRequiredParams(apiName: string, params: ~Object+) : void {
+    for (const key of Object.keys(params)) {
+        const arg = params[key];
+        if (arg == null) {
+            throw new Error('Required parameter ' + key + ' was null or undefined when calling ' + apiName + '.');
+        }
+    }
+}
+
+export public function <T> cleanCopyBody(t : T+, ...properties: string) : ~T {
+    const copy : ~T+ = {};
+    for (const prop in properties) {
+        copy[prop] = t.prop;
+    }
+    return copy;
+}
+
+/**
+ * Default implementation of ApiExecuterI
+ *
+ * The following dependencies are necessary:
+ * - n4js-runtime-esnext
+ * - n4js-runtime-es2015
+ * - n4js-runtime-html5
+ */
+export public class FetchApiExec implements ApiExecuterI {
+    public apiOrigin: string;
+    const jsonTypes = ["application/json", "application/problem+json"];
+
+    @Override
+    public <R, E> async exec(
+        method: string,
+        path: string,
+        pathParams: ~Object+,
+        queryParams: ~Object+,
+        headerParams: ~Object+,
+        payloadContentType: string,
+        body: any+
+    ): Promise<R, Object|ApiError<E>> {
+
+        if (pathParams) {
+            for (const [k, v] of Object.entries(pathParams)) {
+                path = path.replace(`{${k}}`, encodeURIComponent(String(v)));
+            }
+        }
+        const query: string[] = [];
+        if (queryParams) {
+            for (const [k, v] of Object.entries(queryParams)) {
+                query.push(`${k}=${encodeURIComponent(String(v))}`);
+            }
+        }
+
+        let url = `${this.apiOrigin}${path}`;
+        if (query.length) {
+            url += `?${query.join("&")}`;
+        }
+
+        const headers: Object+ = {};
+        if (payloadContentType) {
+            headers["content-type"] = payloadContentType;
+            if (this.constructor.jsonTypes.includes(payloadContentType)) {
+                body = JSON.stringify(body);
+            }
+        }
+        Object.assign(headers, headerParams);
+
+        return await this.<R,E>fetchExec(url, {
+            method,
+            headers,
+            body,
+        });
+    }
+
+    protected <R, E> async fetchExec(url: string, reqInit: RequestInit): Promise<R, Object|ApiError<E>> {
+        const resp = await fetch(url, reqInit);
+
+        if (resp.status !== 204) {
+            const contentType = (resp.headers.get("content-type") || "").split(";")[0];
+            const body = this.constructor.jsonTypes.includes(contentType)
+                ? await resp.json()
+                : await resp.text();
+
+            if (!resp.ok) {
+                await this.handleError(resp, body);
+            }
+            return body as R;
+        }
+        return null;
+    }
+
+    protected <E> async handleError(resp: Response, body): Promise<undefined, ApiError<E>> {
+        throw {body: body};
+    }
+}