How to check if a class in javascript exists

Question

I am defining a class in javascript using this...

// file_a.js
function class_a() {
    this.prop1 = null;
    this.prop2 = null;
}

// file_b.js
var obj = new class_a;
// I need to check here if class_a exists

How can I do this?

Regards

Nitpicky: Classes don't exist in JavaScript :) (only objects (functions are also objects) and some primitive types) — Felix Kling
– Felix Kling, Commented Nov 30, 2010 at 13:13

Quentin · Accepted Answer · 2010-11-30 13:08:51Z

85

if (typeof class_a === 'function')

answered Nov 30, 2010 at 13:08

Quentin

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5 Comments

Olle Härstedt Over a year ago

This won't work when the class has not yet been defined.

Quentin Over a year ago

@OlleHärstedt — If it hasn't yet been defined, then class_a will be undefined so typeof class_a will be the string 'undefined', so the === test will evaluate as false which is the correct answer. It works perfectly.

Olle Härstedt Over a year ago

Correct, my fault. I had an error at a different place that confused me.

Suhail Mumtaz Awan Over a year ago

what if i have class name in string? typeof window['className'] === 'function' will always return false?

Cesar Over a year ago

Be aware that this only works in simple cases when class_a is directly under window. This doesn't work for module1.class1 since module1 might be also undefined.

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