What's the reason to use === instead of == with typeof in Javascript?

Question

Throughout many third-party libraries and best practices blogs/recommendations, etc... it is common to see syntax like this:

typeof x === 'object' (instead of typeof x == 'object') typeof y === 'string' (instead of typeof x == 'string') typeof z === 'function' (instead of typeof x == 'function') 

If the typeof operator already returns a string, what's the need to type check the return value as well? If typeof(typeof(x)) is always string, no matter what x actually is, then == should be sufficient and === unnecessary.

Under what circumstances will typeof not return a string literal? And even if there's some fringe case why is the additional type check being used for object, string, function, etc...

Answer 1

To answer the main question - there is no danger in using typeof with ==. Below is the reason why you may want to use === anyway.

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The recommendation from Crockford is that it's safer to use === in many circumstances, and that if you're going to use it in some circumstances it's better to be consistent and use it for everything.

The thinking is that you can either think about whether to use == or === every time you check for equality, or you can just get into the habit of always writing ===.

There's hardly ever a reason for using == over === - if you're comparing to true or false and you want coercion (for example you want 0 or '' to evaluate to false) then just use if(! empty_str) rather than if(empty_str == false).

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To those who don't understand the problems of == outside of the context of typeof, see this, from The Good Parts:

'' == '0'          // false 0 == ''            // true 0 == '0'           // true  false == 'false'   // false false == '0'       // true  false == undefined // false false == null      // false null == undefined  // true  ' \t\r\n ' == 0    // true 

Answer 2

If the typeof operator already returns a string, what's the need to type check the return value as well? If typeof(typeof(x)) is always string, no matter what x actually is, then == should be sufficient and === unnecessary.

It's subjective. You can just as easily turn this around, and ask, "Why would you use == when you don't expect implicit conversions?" Both work fine here, so use the one you feel expresses your intention better. Try to be consistent within a project.