index-array-by

index-array-by
==============

[![NPM package][npm-img]][npm-url]
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A utility function to index arrays by any criteria.

indexBy(list, keyAccessors, multiItem = true)

Quick start

``js
import indexBy from 'index-array-by';
`
or using a script tag
`html

`

Usage example

Given an array
`js
const people = [
{ name: 'Mary', surname: 'Jane', age: 28 },
{ name: 'John', surname: 'Smith', age: 24 },
{ name: 'John', surname: 'Doe', age: 32 }
];
`

Use indexBy to index it by a given attribute (string type keyAccessor) or any other custom criteria (function type keyAccessor). You can also pass an array of keyAccessors to retrieve a nested object recursively indexed by the multiple keys.

Use the third parameter (multiItem) to indicate whether each key should point to a single item (unadvised if the keys are not unique) or an array of multiple items (default behavior).

`js
indexBy(people, 'surname', false);

// Result:
{
Doe: { name: 'John', age: 32 },
Jane: { name: 'Mary', age: 28 },
Smith: { name: 'John', age: 24 }
}
`

`js
indexBy(people, 'name', true);

// Result:
{
Mary: [ { surname: 'Jane', age: 28 } ],
John: [
{ surname: 'Smith', age: 24 },
{ surname: 'Doe', age: 32 }
]
}
`

`js
indexBy(people, ({ name, surname }) => ${surname}, ${name}, false);

// Result:
{
'Jane, Mary': { name: 'Mary', surname: 'Jane', age: 28 },
'Smith, John': { name: 'John', surname: 'Smith', age: 24 },
'Doe, John': { name: 'John', surname: 'Doe', age: 32 }
}
`

`js
indexBy(people, ['name', 'surname'], false));

// Result:
{
Mary: { Jane: { age: 28 }},
John: { Smith: { age: 24 }, Doe: { age: 32 }}
}
`

`js
indexBy(people, ({ age }) => ${Math.floor(age / 10) * 10}s, true);

// Result:
{
'20s': [
{ name: 'Mary', surname: 'Jane', age: 28 },
{ name: 'John', surname: 'Smith', age: 24 },
],
'30s': [{ name: 'John', surname: 'Doe', age: 32 }]
}
`


The multiItem parameter also accepts a transformation function with the method to reduce multiple items into a single one. In this case, it's keeping only the max age.

`js
indexBy(people, 'name', items => Math.max(...items.map(item => item.age)));

// Result:

{
John: 32,
Mary: 28
}
`


A fourth optional parameter (flattenKeys) (default: false) allows you to receive a flat array structure instead of the default nested format, with each item formatted as { keys: [], vals: }.

`js
indexBy(people, ['name', 'surname'], true, true));

// Result:
[
{ keys: ['Mary', 'Jane'], vals: [{ age: 28 }] },
{ keys: ['John', 'Smith'], vals: [{ age: 24 }] },
{ keys: ['John', 'Doe'], vals: [{ age: 32 }] }
]
``

[npm-img]: https://img.shields.io/npm/v/index-array-by
[npm-url]: https://npmjs.org/package/index-array-by
[build-size-img]: https://img.shields.io/bundlephobia/minzip/index-array-by
[build-size-url]: https://bundlephobia.com/result?p=index-array-by
[npm-downloads-img]: https://img.shields.io/npm/dt/index-array-by
[npm-downloads-url]: https://www.npmtrends.com/index-array-by