Usage

`javascript
var mapKeysAsync = require( '@stdlib/utils-async-map-keys' );
`

#### mapKeysAsync( obj, \[options,] transform, done )

Map keys from one object to a new object having the same values.

`javascript
function transform( key, next ) {
setTimeout( onTimeout, 0 );
function onTimeout() {
next( null, key+':beep' );
}
}

function done( error, out ) {
if ( error ) {
throw error;
}
console.log( out );
// => { 'a:beep': 1, 'b:beep': 2 }
}

var obj = {
'a': 1,
'b': 2
};

mapKeysAsync( obj, transform, done );
`

The next callback accepts two arguments: error and key. The second argument to the next callback is the transformed property name. If a transform function calls the next callback with a truthy error argument, the function stops processing any additional own properties and calls the done callback for error processing.

`javascript
function transform( key, value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
if ( value === 1 ) {
return next( new Error( 'boop' ) );
}
next( null, key+':beep' );
}
}

function done( error ) {
if ( error ) {
console.error( error.message );
// => 'boop'
}
}

var obj = {
'a': 1,
'b': 2
};

mapKeysAsync( obj, transform, done );
`

The function accepts the following options:

- limit: the maximum number of pending invocations at any one time. Default: infinity.
- series: boolean indicating whether to sequentially invoke the transform function for each own property. If true, the function sets options.limit=1. Default: false.
- thisArg: the execution context for fcn.

By default, all properties are processed concurrently, which means that the function does not guarantee completion order. To process each property sequentially, set the series option to true.

`javascript
function transform( key, value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
next( null, key+':beep' );
}
}

function done( error, out ) {
if ( error ) {
throw error;
}
console.log( out );
// => { 'a:beep': 1, 'b:beep': 2 }
}

var obj = {
'a': 1,
'b': 2
};

var opts = {
'series': true
};

mapKeysAsync( obj, opts, transform, done );
`

To limit the maximum number of pending function invocations, set the limit option.

`javascript
function transform( key, value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
next( null, key+':beep' );
}
}

function done( error, out ) {
if ( error ) {
throw error;
}
console.log( out );
// => { 'a:beep': 1, 'b:beep': 2, 'c:beep': 3 }
}

var obj = {
'a': 1,
'b': 2,
'c': 3
};

var opts = {
'limit': 2
};

mapKeysAsync( obj, opts, transform, done );
`

To set the execution context of the transform function, set the thisArg option.

`javascript
function transform( key, value, next ) {
this.count += 1;
setTimeout( onTimeout, value );
function onTimeout() {
next( null, key+':beep' );
}
}

var obj = {
'a': 1,
'b': 2,
'c': 3
};

var context = {
'count': 0
};

var opts = {
'thisArg': context
};

mapKeysAsync( obj, opts, transform, done );

function done( error, out ) {
if ( error ) {
throw error;
}
console.log( out );
// => { 'a:beep': 1, 'b:beep': 2, 'c:beep': 3 }

console.log( context.count );
// => 3
}
`

When invoked, the transform function is provided a maximum of four arguments:

- key: object key.
- value: object value corresponding to key.
- obj: source object.
- next: a callback which should be called once the transform function has finished processing a property.

The actual number of provided arguments depends on function length. If the transform function accepts two arguments, the transform function is provided key and next. If the transform function accepts three arguments, the transform function is provided key, value, and next. For every other transform function signature, the transform function is provided all four arguments.

`javascript
function transform( key, value, obj, next ) {
console.log( 'obj: %s. %s: %d', JSON.stringify( obj ), key, value );
setTimeout( onTimeout, value );
function onTimeout() {
next( null, key+':'+value );
}
}

function done( error, out ) {
if ( error ) {
throw error;
}
console.log( out );
}

var obj = {
'a': 1,
'b': 2
};

mapKeysAsync( obj, transform, done );
/* => e.g.,
obj: {"a": 1, "b": 2}. a: 1
obj: {"a": 1, "b": 2}. b: 2
{ 'a:1': 1, 'b:2': 2 }
*/
`

#### mapKeysAsync.factory( \[options,] transform )

Returns a function which invokes a transform function once for each own property.

`javascript
function transform( key, value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
next( null, key+':beep' );
}
}

function done( error, out ) {
if ( error ) {
throw error;
}
console.log( out );
}

var f = mapKeysAsync.factory( transform );

var obj1 = {
'a': 1,
'b': 2
};

f( obj1, done );
// => { 'a:beep': 1, 'b:beep': 2 }

var obj2 = {
'c': 3,
'd': 4
};

f( obj2, done );
// => { 'c:beep': 3, 'd:beep': 4 }
`

The function accepts the same options as mapKeysAsync().