groupOwn

Group an object's own property values according to an indicator function.

Usage

var groupOwn = require( '@stdlib/utils/group-own' );

groupOwn( obj, [options,] indicator )

Groups an object's own property values according to an indicator function, which specifies which group a value in the input object belongs to.

function indicator( v ) {
    return v[ 0 ];
}
var obj = {
    'a': 'beep',
    'b': 'boop',
    'c': 'foo',
    'd': 'bar'
};

var out = groupOwn( obj, indicator );
// e.g., returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ 'foo' ] }

An indicator function is provided two arguments:

  • value: object value
  • key: object index
function indicator( v, k ) {
    console.log( '%s: %s', k, v );
    return v[ 0 ];
}
var obj = {
    'a': 'beep',
    'b': 'boop',
    'c': 'foo',
    'd': 'bar'
};

var out = groupOwn( obj, indicator );
// e.g., returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ 'foo' ] }

The function accepts the following options:

  • returns: specifies the output format. If the option equals 'values', the function outputs values. If the option equals 'keys', the function outputs keys. If the option equals '*', the function outputs both keys and values. Default: 'values'.
  • thisArg: execution context.

By default, the function returns object values. To return object keys, set the returns option to 'keys'.

function indicator( v ) {
    return v[ 0 ];
}
var obj = {
    'a': 'beep',
    'b': 'boop',
    'c': 'foo',
    'd': 'bar'
};

var opts = {
    'returns': 'keys'
};
var out = groupOwn( obj, opts, indicator );
// e.g., returns { 'b': [ 'a', 'b', 'd' ], 'f': [ 'c' ] }

To return key-value pairs, set the returns option to '*'.

function indicator( v ) {
    return v[ 0 ];
}
var obj = {
    'a': 'beep',
    'b': 'boop',
    'c': 'foo',
    'd': 'bar'
};

var opts = {
    'returns': '*'
};
var out = groupOwn( obj, opts, indicator );
// e.g., returns { 'b': [ [ 'a', 'beep' ], [ 'b', 'boop ], [ 'd', 'bar' ] ], 'f': [ [ 'c', 'foo' ] ] }

To set the indicator execution context, provide a thisArg.

function indicator( v ) {
    this.count += 1;
    return v[ 0 ];
}
var context = {
    'count': 0
};
var opts = {
    'thisArg': context
};
var obj = {
    'a': 'beep',
    'b': 'boop',
    'c': 'foo',
    'd': 'bar'
};
var out = groupOwn( obj, opts, indicator );
// e.g., returns { 'b': [ 'beep', 'boop', 'bar' ], 'f': [ 'foo' ] }

console.log( context.count );
// => 4

Notes

  • Iteration order is not guaranteed, as object key enumeration is not specified according to the ECMAScript specification. In practice, however, most engines use insertion order to sort an object's keys, thus allowing for deterministic iteration.

  • Because iteration order is not guaranteed, result order is not guaranteed.

  • The function determines the list of own enumerable properties before invoking the provided function. Hence, any modifications made to the input object after calling this function (such as adding and removing properties) will not affect the list of visited properties.

  • The value returned by an indicator function should be a value which can be serialized as an object key. As a counterexample,

    function indicator( v ) {
        return {};
    }
    var obj = {
        'a': 'beep',
        'b': 'boop',
        'c': 'foo',
        'd': 'bar'
    };
    var out = groupOwn( obj, indicator );
    // e.g., returns { '[object Object]': [ 'beep', 'boop', 'foo', 'bar' ] }
    

    while each group identifier is unique, all object values resolve to the same group because each group identifier serializes to the same string.

Examples

var randu = require( '@stdlib/random/base/randu' );
var fromCodePoint = require( '@stdlib/string/from-code-point' );
var groupOwn = require( '@stdlib/utils/group-own' );

var key;
var obj;
var out;
var i;

// Generate a random object...
obj = {};
for ( i = 0; i < 100; i++ ) {
    key = fromCodePoint( 97+i );
    obj[ key ] = randu();
}

function indicator( v ) {
    if ( v < 0.5 ) {
        return 'low';
    }
    return 'high';
}

// Compute the groups:
out = groupOwn( obj, indicator );
console.log( out );