Essential Books

Saving this for use in nested functions objects

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One common pitfall is to try and use this in a nested function or an object, where the context has been lost. 

document.getElementById('myAJAXButton').onclick = function(){
    makeAJAXRequest(function(result){
      if (result) { // success
        this.className = 'success';
      }
    })
}

Here the context (this) is lost in the inner callback function. To correct this, you can save the value of this in a variable: 

document.getElementById('myAJAXButton').onclick = function(){
    var self = this;
    makeAJAXRequest(function(result){
      if (result) { // success
        self.className = 'success';
      }
    })
}

ES6 introduced arrow functions which include lexical this binding. The above example could be written like this: 

document.getElementById('myAJAXButton').onclick = function(){
    makeAJAXRequest(result => {
      if (result) { // success
        this.className = 'success';
      }
    })
}
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Context this:
* this with simple objects
* Saving this for use in nested functions objects
* Binding function context
* this in constructor functions
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95 Evaluating JavaScript
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97 How to make iterator usable inside async callback function
98 Context this
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100 Vibration API
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102 Tilde
103 Security issues
104 Get and set CSS custom variables
105 Contributors