# map.groupby <sup>[![Version Badge][npm-version-svg]][package-url]</sup>

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An ESnext spec-compliant `Map.groupBy` shim/polyfill/replacement that works as far down as ES3.

This package implements the [es-shim API](https://github.com/es-shims/api) interface. It works in an ES3-supported environment and complies with the proposed [spec](https://tc39.github.io/proposal-array-grouping/).

Note: Safari 16 implements `Array.prototype.groupToMap`, but it does not work properly with strings containing surrogate pairs.

## Getting started

```sh
npm install --save map.groupby
```

## Usage/Examples

```js
var groupBy = require('map.groupby');
var assert = require('assert');

var arr = [0, 1, 2, 3, 4, 5];
var parity = function (x) { return x % 2 === 0 ? 'even' : 'odd'; };

var results = groupBy(arr, function (x, i) {
    assert.equal(x, arr[i]);
    return parity(x);
});

assert.deepEqual(results, new Map([
    ['even', [0, 2, 4]],
    ['odd', [1, 3, 5]],
]));
```

```js
var groupBy = require('map.groupby');
var assert = require('assert');
/* when Map.groupBy is not present */
delete Map.groupBy;
var shimmed = groupBy.shim();

assert.equal(shimmed, groupBy.getPolyfill());
assert.deepEqual(Map.groupBy(arr, parity), groupBy(arr, parity));
```

```js
var groupBy = require('map.groupby');
var assert = require('assert');
/* when Array#group is present */
var shimmed = groupBy.shim();

assert.equal(shimmed, Map.groupBy);
assert.deepEqual(Map.groupBy(arr, parity), groupBy(arr, parity));
```

## Tests
Simply clone the repo, `npm install`, and run `npm test`

[package-url]: https://npmjs.org/package/map.groupby
[npm-version-svg]: https://versionbadg.es/es-shims/Map.groupBy.svg
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[license-url]: LICENSE
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