Unraveling the mystery of "??=" in Javascript/Typescript code

In a recent TypeScript code snippet, I came across the following:

const arrayAA: Record<
    someSchema['propX'],
    typeof arrayBB
    > = {};
    
for (const varB of arrayBB) {
    (arrayAA[someStringValue] ??= []).push(varB)
}

What is the significance of "??=" in this context?

I have not been able to locate any information about "??=" in the official documentation.

==========

(Addendum after receiving input)

It appears that this question has been asked previously (despite my search for "??=" on StackOverflow).

Thus, could this code be reframed as follows:

if (arrayAA[someStringValue] === undefined || arrayAA[someStringValue] === null) { 
    arrayAA[someStringValue] = []; 
} 
arrayAA[someStringValue].push(varB)

(Special thanks to Ivar)

Answer №1

This feature is called Nullish coalescing assignment (??=).

It specifically assigns a value only when the left operand is null or undefined.

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