Where can I get info on the object parameter syntax for JavaScript functions?

Question

If I want to call a function like this:

moo({ a: 4 });

Normally I'd have to phrase my function definition like this:

function moo(myArgObj) {
    print(myArgObj.a);
}

But this awesome syntax is totally valid in spidermonkey for defining functions:

function moo({ a, b, c }) { // valid syntax!
    print(a); // prints 4
}

What is this feature?

Didn't you miss the property names in that arguments "object declaration"? — Bergi
– Bergi, Commented May 29, 2012 at 19:27

str · Accepted Answer · 2019-12-25 20:54:59Z

7

It's called destructuring. You might find the most info at MDN: Destructuring assignment (especially see Unpacking fields from objects passed as function parameter).

The ECMAScript standards discussion can be found on their wiki page, also interesting might be this blog post at dailyjs.

edited Dec 25, 2019 at 20:54

str

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answered May 29, 2012 at 19:16

Bergi

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5 Comments

Nguyễn Xuân Hoàng Over a year ago

some links died

Bergi Over a year ago

@NguyễnXuânHoàng Thanks, but I'm not sure whether they're dead or just unresponsive (especially the ecmascript wiki is often down). You can find both of them in the internet archive though: [1], [2]

Bergi Over a year ago

@NguyễnXuânHoàng And of course, more than five years later, the drafts have made their way into the standard, and there are much better resources on the topic available today :-)

Bob Over a year ago

destructuring explains how the values of the properties in the passed object get assigned to the properties in the parameter variable but it doesn't explain how those properties can be accessed in the function without qualification - ie. how the whole business works with an unnamed parameter. Any idea where that comes from ?

Bergi Over a year ago

@Bob There are no "properties in the parameter variable". The destructuring targets become local variables in the function, just like normal parameters.

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