JSByte: How to check falsy values with null or undefined in JavaScript

I will be sharing bite sized learnings about JavaScript regularly in this series. Follow along with me as I re-learn JavaScript. This series will cover JS fundamentals, browsers, DOM, system design, domain architecture and frameworks.

console.log( null === undefined )

Rule

An important rule of checking type with null or undefined is that in the equality equation above, the result will be true only if both sides are either null or undefined.

This is helpful in checking against falsy values such as following -

let c;
console.log(c == null);
// true

console.log(c == undefined);
// true

console.log(0 == null);
// false

console.log("" == null);
// false

One caveat: == should be rarely used. This is a good use case for when == can be used. If you are unsure whether to use == or ===, use ===.

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Comments

[Vesa Piittinen]

The only thing I'd like to add is that this is the only case you should ever use == for comparison, and always otherwise use ===. This is a good rule to follow especially if you're working without TypeScript or other type enforcement tool as it greatly reduces bugs caused by mixing datatypes.

[Shruti Kapoor]

Good point! Updating the post.