JavaScript: Testing variables for undefined value; testing whether object is array

Question

Is there a difference between typeof value === "undefined" and value === undefined?
Why did JavaScript need Array.isArray() in ECMAScript 5? Can't I just call value instanceof Array to determine whether an given variable is an array?

Raynos · Accepted Answer · 2011-05-15 02:59:26Z

7

//var value; There is no var declaration. The variable was never declared

// check againts undeclared variables
typeof value === "undefined"; // works

// check againts declared variables with no value
value === undefined; // ReferenceError: value is not defined

There are also issues with undefined = true being valid. But you don't really care about that. No-one is stupid enough to alter undefined globally these days.

I also know of bugs with instanceof being broken. I can't give you the exact reason why Array.isArray is better though.

You will find criticism on instanceof in the JavaScript Garden

If you read this Article It mentions how instanceof does not work across seperate frames / windows / iframes.

Because instanceof checks againts Array and each window has it's own window.Array.

edited May 15, 2011 at 2:59

answered May 15, 2011 at 2:41

Raynos

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2 Comments

FoobarMaximus Over a year ago

Thanks! I just tried var foo; (foo === undefined); in my FF4 JS console and didn't get a reference error. What gives?

Raynos Over a year ago

@FooBarMaximus you get a reference error if you drop var foo ;) typeof works without the var declaration.

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