Use of the identity function in JavaScript

javascript function functional-programming identity use-case
Aadit M Shah picture Aadit M Shah · Jul 14, 2012 · Viewed 20.7k times · Source

I use the identity function in all my JavaScript programs:

function identity(value) {
    return value;
}

The reason is that I often need differentiate between primitives types (undefined, null, boolean, number and string) and object types (object and function) as returned by the typeof operator. I feel using the indentity function for this use case very succuint:

if (new identity(value) == value); // value is of an object type
if (new identity(value) != value); // value is of a primitive type

The identity function is much smaller and simpler than the following code:

function isObject(value) {
    var type = typeof value;
    return type == "object" || type == "function";
}

However on reading my code a friend of mine complained that my hack is misleading and more computationally expensive than the above alternative.

I don't want to remove this function from any of my programs as I believe it's an elegant hack. Then again I don't write programs solely for myself. Is there any other use case for the identity function in JavaScript?

Answer

Tomasz Nurkiewicz picture Tomasz Nurkiewicz · Jul 14, 2012

IMHO:

new identity(value) == value

means absolutely nothing and without extra comment I would have to think for a while to figure out what the intent was. On the other hand:

isObject(value)

is obvious from the very beginning, no matter how it is implemented. Why can't you use your hack inside a function named isObject()?

BTW More suited for http://codereview.stackexchange.com.

Related questions

What object javascript function is bound to (what is its "this")?

I know that inside the function it is this. var func = function { return this.f === arguments.callee; // => true, if bound to some object // => false, if is bound to null, because this.f === undefined } var f = func; // not bound …

Read More
Is this a pure function?

Most sources define a pure function as having the following two properties: Its return value is the same for the same arguments. Its evaluation has no side effects. It is the first condition that concerns me. In most cases, it's …

Read More
How do JavaScript closures work?

How would you explain JavaScript closures to someone with a knowledge of the concepts they consist of (for example functions, variables and the like), but does not understand closures themselves? I have seen the Scheme example given on Wikipedia, but …

Read More