Borrowing Methods in JavaScript

Borrowing Methods in JavaScript. By using call(), apply() and bind(), we can easily borrow methods from different objects without having to inherit from them – a useful tool in a professional JavaScript developer’s toolbox.

In JavaScript, sometimes it’s desirable to reuse a function or method on a different object other than the object or prototype it was defined on. By using call(), apply() and bind(), we can easily borrow methods from different objects without having to inherit from them – a useful tool in a professional JavaScript developer’s toolbox.

Methods from native prototypes

In JavaScript, almost everything you touch is an object except for primitives such as string, number and booleans which are immutable. An Array is a type of object suited to traversing and mutating an ordered list of data, and comes with useful methods on its prototype such as slice, join, push and pop.

A common use case we see with objects is to “borrow” methods from an array as they are both list type data structures. The most common borrowed method is Array.prototype.slice.

function myFunc() {

    // error, arguments is an array like object, not a real array
    arguments.sort();

    // "borrow" the Array method slice from its prototype, which takes an array like object (key:value)
    // and returns a real array
    var args = Array.prototype.slice.call(arguments);

    // args is now a real Array, so can use the sort() method from Array
    args.sort();

}

myFunc('bananas', 'cherries', 'apples');

Borrowing methods is possible due to call and apply allowing us to invoke functions in a different context and is a great way to reuse existing functionality without having to make one object extend from another. An array actually has many methods defined on its prototype that are considered generically reusable, two further examples of which are join and filter:

// takes a string "abc" and produces "a|b|c
Array.prototype.join.call('abc', '|');

// takes a string and removes all non vowels
Array.prototype.filter.call('abcdefghijk', function(val) {
    return ['a', 'e', 'i', 'o', 'u'].indexOf(val) !== -1;
}).join('');

As you can see it’s not just objects who benefit by borrowing methods from an array, strings can too. However, since generic methods are defined on the prototype having to write String.prototype or Array.prototype each time we want to borrow a method is verbose and fast becomes tiresome. An alternative, valid approach that is effectively the same is to use Literals.

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