Another weird operator to save us from infinite if statements.

Falsy

A Falsy is a value considered false in a boolean context. The following values are the falsy values in Javascript

null, undefined, false, NaN, 0, -0, 0n, "", document.all

Truthy

A Truthy value is a value considered true in a boolean context. If values are not falsy, then they’re all truthy.

Nullish

A Nullish value is the value which is either null or undefined, which is always falsy.

&&= Logical AND assignment

This operator assigns the right term to the left term if the left term is truthy.

let truthyValue = 1
let falsyValue = false

truthyValue &&= 2
falsyValue &&= 3

console.log(truthyValue, falsyValue)
// Expected output: 2, false

||= Logical OR assignment

This operator assigns the right term to the left term if the left term is falsy.

let truthyValue = 1
let falsyValue = false

truthyValue ||= 2
falsyValue ||= 3

console.log(truthyValue, falsyValue)
// Expected output: 1, 3

??= Nullish coalescing assignment

This operator assigns the right term to the left term if the left term is nullish.

let truthyValue = 1
let falsyValue = false
let nullishValue = null

truthyValue ??= 2
falsyValue ??= 3
nullishValue ??= "Hola"

console.log(truthyValue, falsyValue, nullishValue)
// Expected output: 1, false, Hola

References

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators