从函数 angular2 调用服务

Question

I'm trying to call a service from a function but i'm getting this error in the console:

Cannot read property 'TestMethod' of undefined

This is my code:

• app.component.ts:

   constructor(public _service: BackendFileCodeService){ } public editor; EditorCreated(quill) { const toolbar = quill.getModule('toolbar'); this.editor = quill; // console.log(quill) toolbar.addHandler('image', this.imageHandler); } imageHandler(value) { // document.querySelector('input.ql-image[type=file]').addEventListener('click', () => { // console.log('Hello'); // }); const ImageInput = document.createElement('input'); ImageInput.setAttribute('type', 'file'); ImageInput.setAttribute('accept', 'image/png, image/gif, image/jpeg, image/bmp, image/x-icon'); ImageInput.classList.add('ql-image'); ImageInput.click(); ImageInput.addEventListener('change', () => { const file = ImageInput.files[0]; if (ImageInput.files != null && ImageInput.files[0] != null) { this._service.TestMethod('Hello'); } }); }

• BackendFileCodeService:

   import { Injectable } from '@angular/core'; @Injectable() export class BackendFileCodeService { constructor() { } TestMethod(test){ return test; } }

I'm trying to call the service inside the function called imageHandler specifically in the

ImageInput.addEventListener

but i'm getting the error mentioned up, i tried to call the service from outside the imageHandler function and every this works as expected.

Note: the service is console log 'hello' as test.

Answer 1

Using fat arrow syntax for your imageHandler function should resolve the issue.

imageHandler(value) {...}
...
imageHandler = (value) => { ... }

If you can not use fat arrow syntax, you would need to create a variable to capture the component scope, the most common way is declaring a variable and assigning to this,

self = this;
imageHandler(value) {
 ....
 self._service.TestMethod('Hello');
 ....
}

I see you are aalready using fat arrow syntax below, which is good else you would also need to capture scope for imageHandler .

ImageInput.addEventListener('change', () => {

Read more about function scopes in typescript here ,

this and arrow functions

In JavaScript, this is a variable that's set when a function is called. This makes it a very powerful and flexible feature, but it comes at the cost of always having to know about the context that a function is executing in. This is notoriously confusing, especially when returning a function or passing a function as an argument.