CODEWITHSUNDEEP
javascript
Liglo App
Liglo App

Reputation: 3819

Accessing "this" in Function.prototype not possible?

I am trying to extend the function prototype to return a singleton version of a function.

Function.prototype.once = function() { 
  var called = false, memo; 
  return function() { 
        console.log('AP', this); 
        if (!called) memo = this.apply(this, arguments); 
        called = true; 
        return memo; 
  } 
}

The console logs the window object. Why is this != current function? And how to solve this?

Upvotes: 1

Views: 77

Answers (3)

user229044
user229044

Reputation: 239260

You can't "close over" this, so you need to either resort to the old var self = this trick (ie, obtain a reference to this in a variable which can be closed over) or simply bind your function:

return function() { 
    console.log('AP', this); 
    if (!called) memo = this.apply(this, arguments); 
    called = true; 
    return memo; 
}.bind(this);

Upvotes: 7

Florent
Florent

Reputation: 12420

You have to bind to context on the anonymous function.

Function.prototype.once = function() { 
  var called = false, memo; 
  return (function() { 
    console.log('AP', this); 
    if (!called) memo = this.apply(this, arguments); 
    called = true; 
    return memo; 
  }).bind(this);
}

Upvotes: 1

Alnitak
Alnitak

Reputation: 339786

Sure, it's possible, but your inner function creates a new context so the this inside it is not the same as the this outside.

Just create an external reference to the original function:

Function.prototype.once = function() {
    var f = this;   // use 'f' in the inner function
     ...
}

NB: depending on your intent you may also have the same issue with arguments.

Upvotes: 4

Related Questions