Usage

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

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

Merges and extends a target object.

`javascript
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.

`javascript
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.

`javascript
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][@stdlib/utils/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.

`javascript
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.

`javascript
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][@stdlib/utils/copy].

`javascript
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 );
// => false
`

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

`javascript
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.

`javascript
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.

`javascript
function strategy( a, b, key ) {
/* Parameters:
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.

`javascript
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'
}
*/
`