keyBy

Convert a collection to an object whose keys are determined by a provided function and whose values are the collection values.

Installation

npm install @stdlib/utils-key-by

Usage

var keyBy = require( '@stdlib/utils-key-by' );

keyBy( collection, fcn[, thisArg ] )

Converts a collection to an object whose keys are determined by a provided function and whose values are the collection values.

function toKey( value ) {
    return value.a;
}

var arr = [
    { 'a': 1 },
    { 'a': 2 }
];

var out = keyBy( arr, toKey );
// returns { '1': { 'a': 1 }, '2': { 'a': 2 } }

The invoked function is provided two arguments:

• value: collection element
• index: collection index

To set the function execution context, provide a thisArg.

function toKey( value, index ) {
    this.sum += value;
    this.count += 1;
    return index;
}

var arr = [ 1, 2, 3, 4 ];

var context = {
    'sum': 0,
    'count': 0
};

var out = keyBy( arr, toKey, context );
// returns { '0': 1, '1': 2, '2': 3, '3': 4 }

var mean = context.sum / context.count;
// returns 2.5

Notes

• A collection may be either an Array, Typed Array, or an array-like Object (excluding strings and functions).
• If more than one element in a collection resolves to the same key, the key value is the collection element which last resolved to the key.
• Object values are shallow copies.

Examples

var keyBy = require( '@stdlib/utils-key-by' );

var arr;
var obj;
var i;

function toKey( value ) {
    return value.name;
}

arr = new Array( 100 );
for ( i = 0; i < arr.length; i++ ) {
    arr[ i ] = {
        'name': 'v'+i,
        'value': i
    };
}

obj = keyBy( arr, toKey );
console.log( obj );

See Also

• @stdlib/utils-for-each: invoke a function for each element in a collection.

Notice

This package is part of stdlib, a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop stdlib, see the main project repository.

Community

License

See LICENSE.

Copyright

Copyright © 2016-2024. The Stdlib Authors.