从同一对象内的另一个函数引用一个对象内的函数。 JavaScript对象文字形式

Question

In the code below when I try to call promptUpdater function from inside userInputOn function in the ArrayRotation object, I get a TypeError: Cannot read property promptUpdater of undefined.

I guess, its the last line of the code rotationTaskObj.userInputOn(); where I need to correct reference, but I am not able to do it.

'use strict';

 var ArrayRotation = {
   arr : [],

   promptUpdater : function() {
     //code
   },

   userInputOn : function() {
     return new Promise(function(resolve, reject) {
       this.promptUpdater(); //correct way to reference promptUpdater fn?
     });
    },
  }

 let rotationTaskObj = Object.create(ArrayRotation);

 let userInputPr = rotationTaskObj.userInputOn(); //My guess, I need to create correct refernce here, not able to get how to do that?

What is the correct way to reference promptUpdater function?

Answer 1

Your problem is where you are trying to call this.promptUpdater() , this isn't what you think it is. this is scoped to the current function, which is your function(resolve, reject) , not your object.

Traditional function method with .bind

If you don't want to use arrow functions (you should, but I won't judge :-)), you can set a function's this value using the bind() method:

To change userInputOn to bind it's this to it's parent's this :

   userInputOn : function() {
       return new Promise(function(resolve, reject) {
           this.promptUpdater(); //correct way to reference promptUpdater fn?
       }.bind(this));
   },

Arrow function method

Change

 return new Promise(function(resolve, reject) {

To

 return new Promise((resolve, reject) => {

Arrow functions don't have a this like normal functions, they pick up the this from the hosting function.

Here's a fiddle:" https://jsfiddle.net/jmbldwn/fpa9xzw0/5/

Answer 2

This is an issue with the this reference getting "lost" inside the Promise's callback function. There are a number of fixes, but the easiest as of ES6 is to use an arrow function instead:

new Promise ((resolve, reject) => {this.promptUpdateUser();})