How to use Bind in javaScript

2 minute read

TIL about the bind method in javaScript and how to use it.

Bind is used for “binding” another object context to a method call. In other words, you can swap out the object that this references in a method by calling bind on the method and passing in a context.

Example 1:

Let’s look at the following example (not using bind yet):

var Brian = {
	firstName: 'Brian',
	lastName: 'Ambielli',
	fullName: function() {
		return this.firstName + ' ' + this.lastName;
	}
};

var fName = Brian.fullName; //this assigns the object's method to the global variable fName

console.log(fName()); //this will print 'undefined undefined'

The result of the console.log is ‘undefined undefined’! Why is that? Because we “borrowed” the method from the Brian object, but we did not borrow its context! Context is what the this in a method refers to, and it is defined by default as the object in which the method is invoked.

A call to Brian.fullName() will call the fullName method with the Brian context, and will appropriately return ‘Brian Ambielli’. However, when calling the method in the global scope (as we do when calling fName() above) the context is the global “window” object.

Example 2:

var Brian = {
	firstName: 'Brian',
	lastName: 'Ambielli',
	fullName: function() {
		return this.firstName + ' ' + this.lastName;
	}
};

var fName = Brian.fullName; //this assigns the object's method to the global variable fName

var firstName = 'Jim';
var lastName = 'Frank';

console.log(fName()); //this will print 'Jim Frank'

In this case, the global object has firstName and lastName properties attached to it already, so when the function references this.firstName and this.lastName those properties will actually be defined (on window). But what if we just want to call fName global scope with the context of the borrowed method’s original object?

This is where bind comes in.

Example 3:

var Brian = {
	firstName: 'Brian',
	lastName: 'Ambielli',
	fullName: function() {
		return this.firstName + ' ' + this.lastName;
	}
};

var fName = Brian.fullName.bind(Brian); //this binds the Brian context to the fullName method

var firstName = 'Jim';
var lastName = 'Frank';

console.log(fName()); //this will print 'Brian Ambielli'

We did it! By binding the object Brian to the fullName method when we assigned it to the global variable fName, we were able to preserve the context for that copy of the method.

Bind is useful when we want to permanently “borrow” methods from other objects, and specify the context in which we want the copy to operate.

If we do not want to permanently “borrow” a method (assigning the copy to a new property in our intended scope) javaScript has other ways to call or apply methods in place without needing to make a copy.

For another day…

Extra Example:

Apparently it is possible to bind and invoke the bound function in place.

var Brian = {
	firstName: 'Brian',
	lastName: 'Ambielli',
	fullName: function() {
		return this.firstName + ' ' + this.lastName;
	}
};

var fName = Brian.fullName;

var firstName = 'Jim';
var lastName = 'Frank';

console.log(fName.bind(Brian)()); //this will print 'Brian Ambielli'

The last line binds fName to the Brian context, and then invokes it immediately. This effectively mimics what call does. Not sure if this is at all good practice, and I feel like using call is preferred. EDIT: According to SO it isn’t “bad” practice per-say…you are basically doing the same thing that call does.

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