Node.js Firebase函数获取文本响应错误“；JSON中的意外标记H“；

我试图找出这个错误：

"error: SyntaxError: Unexpected token H in JSON at position 0 at JSON.parse (<anonymous>) at XMLHttpRequest.onLoad (http://localhost:4200/vendor.js:8646:51) at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (http://localhost:4200/polyfills.js:2743:31) at Object.onInvokeTask (http://localhost:4200/vendor.js:38114:33) at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (http://localhost:4200/polyfills.js:2742:36) at Zone.push../node_modules/zone.js/dist/zone.js.Zone.runTask (http://localhost:4200/polyfills.js:2510:47) at ZoneTask.push../node_modules/zone.js/dist/zone.js.ZoneTask.invokeTask [as invoke] (http://localhost:4200/polyfills.js:2818:34) at invokeTask (http://localhost:4200/polyfills.js:3862:14) at XMLHttpRequest.globalZoneAwareCallback (http://localhost:4200/polyfills.js:3888:17)
text: "Hello from Firebase!"

下面是我对函数的调用：

exports.helloWorld = functions.https.onRequest((req, res) => {
  cors(req, res, () => {
    res.send("Hello from Firebase!");
  });
});

public sendHttpRequestTest(){
    let url = `my_link_to_functions/helloWorld`;
    const req = new HttpRequest("GET",url);

    this.http.request(req).toPromise()
        .then( res => {
          console.log(res);
        })
        .catch( err => {
          console.log(err);
        })
  }

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我通过将其作为JSON发送来修复它，如下所示：

res.send(JSON.stringify("Hello from Firebase!"));

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为什么对

url

变量使用模板文本？