CODEWITHSUNDEEP
javascript
Mitar
Mitar

Reputation: 7050

What does this refer to and why?

In this example:

var A = {test: 1, foo: function() { return this.test }}

Why A.foo() returns 1 (at least in node.js)? I thought this will be bound to external caller this, no?

Upvotes: 0

Views: 144

Answers (2)

Brad
Brad

Reputation: 163438

When you call A.foo(), this within foo() is set to the object A, as that is what you called the function on. Therefore, this.test has a value of 1.

You can change what this is referenced to using .call() or .apply().

A.foo.call(newThisValue);

As for why... this gives you great flexibility. You may have a function that acts on this to do something, and the way JavaScript is built allows you to apply that function to any object in a specific way. It is a bit hard to describe, but it does come in handy in situations such as inheritance. See also: http://trephine.org/t/index.php?title=JavaScript_call_and_apply

Upvotes: 5

hugomg
hugomg

Reputation: 69954

In Javascript whenever you call a function using obj.method() notation, this will be bound to obj.

You can work around this by splitting the call into two separate steps:

var f = A.foo;
f(); // "this" will not be A in this case.

Or, abusing the comma operator:

(17, x.f)()

Upvotes: 1

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