Merge

Merge and extend objects.

Usage

var merge = require( '@stdlib/utils/merge' );

merge( target, source1[, source2[,...,sourceN]] )

Merges and extends a target object.

var target = {
    'a': 'beep'
};
var source = {
    'a': 'boop',
    'b': 'bap'
};

var out = merge( target, source );
/* returns
    {
        'a': 'boop',
        'b': 'bap'
    }
*/

The function supports merging multiple source objects.

var target = {
    'a': 'beep'
};
var source1 = {
    'b': 'boop'
};
var source2 = {
    'c': 'cat'
};

var out = merge( target, source1, source2 );
/* returns
    {
        'a': 'beep',
        'b': 'boop',
        'c': 'cat'
    }
*/

merge.factory( options )

Returns a custom merge function for merging and extending objects.

var opts = {
    'level': 100,
    'copy': true,
    'override': true,
    'extend': true
};

var m = merge.factory( opts );

The function accepts the following options:

  • level: limits the merge depth. The default merge strategy is a deep (recursive) merge. Default: +infinity.
  • copy: boolean indicating whether to deep copy merged values. Deep copying prevents shared references and source object mutation. Default: true.
  • override: defines the merge strategy. If true, source object values will always override target object values. If false, source values never override target values (useful for adding, but not overwriting, properties). To define a custom merge strategy, provide a function. Default: true.
  • extend: boolean indicating whether new properties can be added to the target object. If false, only shared properties are merged. Default: true.

The default merge is a deep (recursive) merge.

var m = merge.factory( {} );

var target = {
    'a': {
        'b': {
            'c': 5
        },
        'd': 'beep'
    }
};
var source = {
    'a': {
        'b': {
            'c': 10
        }
    }
};

var out = m( target, source );
/* returns
    {
        'a': {
            'b': {
                'c': 10
            },
            'd': 'beep'
        }
    }
*/

To limit the merge depth, set the level option.

var m = merge.factory({
    'level': 2
});

var target = {
    '1': {
        'a': 'beep',
        '2': {
            '3': null,
            'b': [ 5, 6, 7 ]
        }
    }
};

var source = {
    '1': {
        'b': 'boop',
        '2': {
            '3': [ 1, 2, 3 ]
        }
    }
};

var out = m( target, source );
/* returns
    {
        '1': {
            'a': 'beep',
            'b': 'boop',
            '2': {
                '3': [ 1, 2, 3 ]
            }
        }
    }
*/

By default, merged values are deep copied.

var m = merge.factory( {} );

var target = {
    'a': null
};
var source = {
    'a': {
        'b': [ 1, 2, 3 ]
    }
};

var out = m( target, source );

console.log( out.a.b === source.a.b );
// returns false

To allow shared references, set the copy option to false.

var m = merge.factory({
    'copy': false
});

var target = {};

var source = {
    'a': [ 1, 2, 3 ]
};

var out = m( target, source );

var bool = ( out.a === source.a );
// returns true

To prevent existing properties from being overridden, set the override option to false.

var m = merge.factory({
    'override': false
});

var target = {
    'a': 'beep',
    'b': 'boop'
};

var source = {
    'a': null,
    'c': 'bop'
};

var out = m( target, source );
/* returns
    {
        'a': 'beep',
        'b': 'boop',
        'c': 'bop'
    }
*/

Alternatively, to define a custom merge strategy, set the override option to a function.

function strategy( a, b, key ) {
    // a => target value
    // b => source value
    // key => object key
    if ( key === 'a' ) {
        return b;
    }
    if ( key === 'b' ) {
        return a;
    }
    return 'bebop';
}

var m = merge.factory({
    'override': strategy
});

var target = {
    'a': 'beep',
    'b': 'boop',
    'c': 1234
};

var source = {
    'a': null,
    'b': {},
    'c': 'bop'
};

var out = m( target, source );
/* returns
    {
        'a': null,
        'b': 'boop',
        'c': 'bebop'
    }
*/

To prevent non-existent properties from being added to the target object, set the extend option to false.

var m = merge.factory({
    'extend': false
});

var target = {
    'a': 'beep',
    'b': 'boop'
};

var source = {
    'b': 'hello',
    'c': 'world'
};

var out = m( target, source );
/* returns
    {
        'a': 'beep',
        'b': 'hello'
    }
*/

Notes

  • The target object is mutated.

    var target = {
        'a': 'beep'
    };
    var source = {
        'b': 'boop'
    };
    
    var out = merge( target, source );
    
    console.log( out === target );
    // returns true
    
    console.log( target.b );
    // returns 'boop'
    

    To return a new object, provide an empty object as the first argument.

    var target = {
        'a': 'beep'
    };
    var source = {
        'b': 'boop'
    };
    
    var out = merge( {}, target, source );
    
    console.log( out === target );
    // returns false
    
  • Only plain JavaScript objects are merged and extended. The following values/types are either deep copied or assigned:

    • Boolean
    • String
    • Number
    • Date
    • RegExp
    • Array
    • Int8Array
    • Uint8Array
    • Uint8ClampedArray
    • Init16Array
    • Uint16Array
    • Int32Array
    • Uint32Array
    • Float32Array
    • Float64Array
    • Buffer (Node.js)
    • Set
    • Map
    • Error
    • URIError
    • ReferenceError
    • SyntaxError
    • RangeError
  • Support for deep merging class instances is inherently fragile.

  • Number, String, or Boolean objects are merged as primitives.

  • Functions are not deep copied.


Examples

var merge = require( '@stdlib/utils/merge' );

var target;
var source;
var out;

target = {
    'a': 'beep',
    'b': 'boop',
    'c': {
        'c1': 'woot',
        'c2': false,
        'c3': {
            'c3a': [ 1, 2 ],
            'c3b': null
        }
    },
    'd': [ 1, 2, 3 ]
};

source = {
    'b': 3.141592653589793,
    'c': {
        'c1': 'bap',
        'c3': {
            'c3b': 5,
            'c3c': 'bop'
        },
        'c4': 1337,
        'c5': new Date()
    },
    'd': [ 4, 5, 6 ],
    'e': true
};

out = merge( {}, target, source );
/* returns
    {
        'a': 'beep',
        'b': 3.141592653589793,
        'c': {
            'c1': 'bap',
            'c2': false,
            'c3': {
                'c3a': [ 1, 2 ],
                'c3b': 5,
                'c3c': 'bop'
            },
            'c4': 1337,
            'c5': <Date>
        },
        'd': [ 4, 5, 6 ],
        'e': true
    }
*/