time-to-botec/squiggle/node_modules/@stdlib/utils/async/map-keys/README.md

mapKeysAsync

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

Usage

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.

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.

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.

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.

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.

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.

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.

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().

Notes

• If a provided function calls the next callback with a truthy error argument, the function suspends execution and immediately calls the done callback for subsequent error handling.
• If provided an empty object, the function calls the done callback with an empty object.
• Key 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 iteration order.
• Key insertion order is not guaranteed.
• The value returned by a transform function should be a value which can be serialized as an object key.
• The function only maps own properties. Hence, the function does not map inherited properties.
• The function shallow copies key values.
• Neither mapKeysAsync nor the function returned by the factory method guarantee asynchronous execution. To guarantee asynchrony, wrap the done callback in a function which either executes at the end of the current stack (e.g., nextTick) or during a subsequent turn of the event loop (e.g., setImmediate, setTimeout).

Examples

var resolve = require( 'path' ).resolve;
var readFile = require( '@stdlib/fs/read-file' );
var mapKeysAsync = require( '@stdlib/utils/async/map-keys' );

var files = {
    'file1': resolve( __dirname, 'package.json' ),
    'file2': resolve( __dirname, 'README.md' )
};

function read( key, value, next ) {
    var opts = {
        'encoding': 'utf8'
    };
    readFile( value, opts, onFile );

    function onFile( error ) {
        if ( error ) {
            return next( error, key+':unreadable' );
        }
        next( null, key+':readable' );
    }
}

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

mapKeysAsync( files, read, done );