How can I pass a function that has a reference to the object which passed the function in Javascript

Question

I have the following "class" function in javascript:

function AI() {   
   // this class is responsible for managing AI behavior

   var action;    // this is a private variable 
                  // that will be used to point to a function

  this.setAction = function(an_action) {
       action = an_action;     //this function will receive reference 
  }                            // to another function ( an_action() )

  this.update = function() {
      action();                 // this line will execute the passed-in function
  }
}

===

function Player() {
    this.x = 100;
    this.y = 200;
    ...
    this.brain = new AI();   // an instance of AI class to manage Player actions

    this.brain.setAction(idle);  // idle is a function defined below
    ...
    this.update = function() {
       // here we might move the player's location (x,y)
       this.brain.update();   // this line will call the current (action) 
                              // which is a reference to idle function

    }

   this.draw = function() {
       // here I will draw the player at x,y
   }

   function idel() {
      this.xSpeed = 0;   // the player does not move
      ...

   }

   function jump() {
     this.y += 4;   // or any logic that makes the player jump
     ...            // 
     this.brain.setAction(idle);   //after jumping is done, go back to idle
   }
}

I am basically having an instance of Player that has a public variable (an instance of AI class) which is a Finite State Machine model to control the action of the player.

The AI instance brain is responsible for calling whichever function that is passed to it by the owner player object. The function is passed correctly to the AI class, however, the definition of the action function once called by the AI object does not have any reference to the object which passed the function, and so, any reference to this in that function gets evaluated to undefined.

How can I pass a function to an object with a reference to the object which sent the function?

are you looking for `my_func.bind(this)` https://snook.ca/archives/javascript/javascript_pass — marcusshep, Nov 16 '16 at 15:09

Alejandro C. · Accepted Answer · 2016-11-16T15:10:35.193

1

Use .bind() to set the context (this) of a function at call time. For example: this.brain.setAction(idle.bind(this))

edited Nov 16 '16 at 15:10

answered Nov 16 '16 at 15:09

Alejandro C.

3,771
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18

Can you elaborate more? which part should I change in my code? – Ahmad Nov 16 '16 at 15:10
Updated. Basically, wherever you're passing a function, instead of just passing that function, pass `myfunc.bind(this)` – Alejandro C. Nov 16 '16 at 15:11
Excellent. That did the trick, and now the function has a reference to the sender object.. Thanks – Ahmad Nov 16 '16 at 15:20

How can I pass a function that has a reference to the object which passed the function in Javascript

1 Answers1