babel-plugin-extensible-destructuring

Babel plugin that enables extensible destructuring, inspired by vacuumlabs/es-proposals.

Install

npm install --save-dev babel-plugin-extensible-destructuring
npm install --save extensible-runtime

Usage (for version 4.x.x and Babel 6)

Add the plugin to your .babelrc configuration:

{
  "presets": ["es2015"],
  "plugins": ["extensible-destructuring"]
}

Or directly from code:

var babel = require("babel-core");
babel.transform("code", {
    presets: ['es2015'],
    plugins: ['extensible-destructuring']
})

What it does

The plugin gives you more explicit control of what exactly happens in the process of destructuring. The plugin transforms all destructuring assignments such as:

var {a = 10} = o

to:

var a = __extensible_get__(o, a, 10)

Function __extensible_get__ gets automatically required from extensible-runtime package (this is a separate package and needs to be installed alongside this one. This package currently defines three different versions of __extensible_get__ you can choose from using the impl plugin option (see the section Plugin Options below).

• normal: standard ES6 compatible: no magic here
• immutable: the one that you can use with Immutable.js to destructure its Maps and Lists
• safe: prevent returning values from being undefined. Also includes features of immutable

Option safe is the default one. Typically, there is no reason to use this plugin with normal option.

Check out the example folder for a working example; this is a standalone npm package with all the configuration necessary.

Plugin Options

In case you don't know, babel allows to specify options for each plugin. The syntax looks such as:

{
  "presets": ["es2015"],
  "plugins": [["extensible-destructuring", {"mode": "optout", "impl": "safe"}]]
}

Semantics of these options is:

mode

either 'optin' or 'optout'. In optin (default) mode, the destructuring assignments are transformed only if "use extensible" directive is present at the beginning of the file. OTOH, in optout mode the destructuring assignments are transformed unless you use "use !extensible" directive at the beginning of the file. You can change (default) optin in your .babelrc (note that plugins now becomes nested array)

This machinery is cool for already existing bigger projects in which you may want to start using safe mode gradually: You can use safe destructuring, but optin only those files, that are already written with no-undefined-policy in mind.

impl

Either 'normal', 'immutable' or 'safe'; specifies what version of __extensible_get__ to use.

Immutable destructuring example

Use .babelrc such as:

{
  "presets": ["es2015"],
  "plugins": [["extensible-destructuring", {"mode": "optout", "impl": "immutable"}]]
}

This code works as expected:

import {fromJS} from 'immutable';
const map = fromJS({author: {name: {first: "John", last: "Doe"}, birthdate: "10-10-2010"}});
const {author: {name: {first, last}, birthdate}} = map;

Safe destructuring example

Use .babelrc such as:

{
  "presets": ["es2015"],
  "plugins": [["extensible-destructuring", {"mode": "optout", "impl": "safe"}]]
}

Now, the code:

import {fromJS} from 'immutable';
const map = {a: 10, b: 20}
const {a, b, c} = map;

logs the error to the console:

Error: Key Error: object with keys [ "a", "b" ] does not contain property c

On the other hand

const {a, b, c = null} = map;

is perfectly OK.

Differences from version 3

• no need to patch your code at the entry-point, no need to define global __extensible_get__. We see this as a bad practice and this was the most relevant reason for doing version 4.

• option default was renamed to normal (default is sort-of reserved keyword when dealing with ES6 import/export)

• 'polyfill' renamed to 'runtime', which is much more accurate naming

Using your own implementation of __extensible_get__

Although we believe the existing implementations of __extensible_get__ are quite sufficient, you still might want to use your own. In such a case:

In the .babelrc, set {impl: "mymyget"}, mymyget being the name of your newly created resolver. Then in the entry-point of your code you monkey-patch it inside the package (yes, this makes it accessible also for other parts of your project):

var extensibleRuntime = require('extensible-runtime');
const mymyget = () => ...;
extensibleRuntime.mymyget = mymyget;

Under The Hood

The plugin uses global __extensible_get__ function to destructure the assignment. The number of calls to __extensible_get__ is minimized, so if one writes:

const {a: {b: {c, d, e}}} = map;

plugin uses the unique temporary variable to get result such as:

var __extensible_get__ = require('extensible-runtime').safe
var _tmp = __extensible_get__(__extensible_get__(map, "a"), "b");
var c = __extensible_get__(_tmp, "c");
var d = __extensible_get__(_tmp, "d");
var e = __extensible_get__(_tmp, "e");