Constant declaration with block

Question

Recently I was looking into Firefox Add-on Builder SDK sources, and stumbled on such constants declaration:

const { getCodeForKey, toJSON } = require("../../keyboard/utils");

I could find information about CommonJS Modules, but left part of this assignment slightly confuses me, since it must be language specific, and I couldn't google anything on that.

Can someone point me to some specification/draft that explains what's going on here?

Answer 1

This is a destructuring assignment, something that is currently only implemented by the SpiderMonkey JavaScript engine which is used by Firefox. Here is how it works with arrays:

// Destructuring assignment
[a, b] = foo;

// Equivalent code
a = foo[0];
b = foo[1];

And here is how it works with objects:

// Destructuring assignment
{a, b} = foo;

// Equivalent code
a = foo.a;
b = foo.b;

A slightly more elaborate example:

// Destructuring assignment
{name: a, address: {line1: b}} = foo;

// Equivalent code
a = foo.name;
b = foo.address.line1;

So your code example is equivalent to:

var utilsExports = require("../../keyboard/utils");
const getCodeForKey = utilsExports.getCodeForKey;
const toJSON = utilsExports.toJSON;

It is merely a more convenient way to write it.