Remembering what 'this' refers to - the A, B, C, D method

Posted by Krzysztof Zbiciński on 2016-09-04.

The this reference and how it can dynamically change in a runtime is one of the characteristic features of JavaScript. As far as I know, there is no analogy of this behavior in any other popular programming language.

Because of its uniqueness, it’s no surprise that people are often confused by this mechanism and are not quite sure what this is going to refer to in a particular case.



Just another happy JavaScript developer.

However, there are only four rules that compiler follows when resolving the this reference. For easier remembering, I put them in a sequence of A, B, C, D. The earlier the rule, the bigger importance and priority it has.

A - A new object is being constructed

If the function was called with the new keyword, then this refers to an object constructed using this function.

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function whatIsThis() {
console.log(this);
}
new whatIsThis(); // logs out whatIsThis{}

B - context was BOUND using bind, call or apply

If the function was bound to a context using the bind method or was executed using call or apply methods, then this refers to an object passed as an argument to these functions.

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whatIsThis.bind(2)(); // 2
whatIsThis.call(2); // 2
whatIsThis.apply(2); // 2
// but:
var temp = whatIsThis.bind(2);
new temp(); // whatIsThis{} - the A rule has precedence

C - CONTAINING object

If the function called was conatined within an object, then this refers to the containing object.

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var foo = {whatIsThis: whatIsThis};
foo.whatIsThis(); // foo
// but:
foo.whatIsThis.apply(2); // 2 - the B rule has precendence

D - fallback to the DEFAULT behavior

If answers to all of the rules above are negative, then this refers to the global object or to undefined if strict mode is enabled.

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whatIsThis(); // window, process or undefined

And that’s pretty much it. :) You can check this behavior by yourself, using
a fiddle I’ve created. It contains example code which shows how the this reference changes according to the mentioned rules.

Photo by Diomari Madulara on Unsplash.


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