JarCastingindex-array-by
A utility function to index arrays by any criteria.
indexBy(list, keyAccessors, multiItem = true)
Quick start
import indexBy from 'index-array-by';
or
const indexBy = require('index-array-by');
or even
<script src="//unpkg.com/index-array-by"></script>
Usage example
Given an array
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).
indexBy(people, 'surname', false);
// Result:
{
Doe: { name: 'John', age: 32 },
Jane: { name: 'Mary', age: 28 },
Smith: { name: 'John', age: 24 }
}
indexBy(people, 'name', true);
// Result:
{
Mary: [ { surname: 'Jane', age: 28 } ],
John: [
{ surname: 'Smith', age: 24 },
{ surname: 'Doe', age: 32 }
]
}
indexBy(people, ({ name, surname }) => `${surname}, ${name}`, false);
// Result:
{
'Jane, Mary': { name: 'Mary', surname: 'Jane', age: 28 },
'Smith, John': { name: 'John', surname: 'Smith', age: 24 },
'Doe, John': { name: 'John', surname: 'Doe', age: 32 }
}
indexBy(people, ['name', 'surname'], false));
// Result:
{
Mary: { Jane: { age: 28 }},
John: { Smith: { age: 24 }, Doe: { age: 32 }}
}
indexBy(people, ({ age }) => `${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.
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: [<ordered unique keys for the item>], vals: <single or multiple item> }.
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 }] }
]