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Nullish Coalescing Operator

codefinity profile image Manav Misra ・2 min read

?? is part of ES 2021. Unless you have [Babel] integrated into your project, it will probably not work! Alternatively, just use your 'browser dev tools' (e.g. updated Chrome/FF/Edge).

Using `??` in Chrome Dev Tools


Short-Circuiting of Logical Operator OR - ||

To appreciate the nullish coalescing operator - ??, you should understand:

Specifically, the || short-circuit where if the left-hand side operand is 'truthy,' or can be implicitly coerced to be true, the right-hand side is not evaluated.

For example, 3 || 4, 4 will not be evaluated because, 3, being a 'non-zero' number will be implicitly coerced to true.

Similarly, in "mark" || "anonymous"; "mark", being a non-empty string will be treated as true and there is no need to consider "anonymous".

Summarily, if the left-hand side operand evaluates to true - namely if it's not one of these...

  • 0
  • ""
  • null
  • undefined
  • false

...then the right-hand side operand will not be looked at.

const name = 'Mark Galloway';

// Since 'name' is a 'truthy' string, let's use that one!
const msg = `Hello, ${name || "anon"}`
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Fine, But What About ??

It's quite simple really...it's all exactly the same as πŸ‘†πŸ½ but only null and undefined are considered falsey

Examples

As per MDN:

const foo = undefined ?? 'default string'; // 'foo' will be 'default string'

const buzz = null ?? 'Not null!'; // 'buzz' will be 'not null!'

const baz = 0 ?? 42; // 'baz' will be '0'

const bar = false ?? 42; // 'bar' will be 'false'
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πŸ€·πŸ½β€β™‚οΈ - I don't really have much else to say on this one πŸ‘οΈβ€πŸ—¨οΈ. If you understand short circuiting of || then just trim the 'truthy' - 'falsey' rules down to only be: null or undefined.

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