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* @license Apache-2.0
*
* Copyright (c) 2018 The Stdlib Authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
'use strict';
// MODULES //
var isFunction = require( '@stdlib/assert/is-function' );
var format = require( '@stdlib/string/format' );
// MAIN //
/**
* Function composition.
*
* @param {...Function} f - functions to compose
* @throws {Error} must provide more than one argument
* @throws {TypeError} must provide functions
* @returns {Function} composite function
*
* @example
* function a( x, next ) {
* setTimeout( onTimeout, 0 );
* function onTimeout() {
* next( null, 2*x );
* }
* }
*
* function b( x, next ) {
* setTimeout( onTimeout, 0 );
* function onTimeout() {
* next( null, x+3 );
* }
* }
*
* function c( x, next ) {
* setTimeout( onTimeout, 0 );
* function onTimeout() {
* next( null, x/5 );
* }
* }
*
* var f = composeAsync( c, b, a );
*
* function done( error, result ) {
* if ( error ) {
* throw error;
* }
* console.log( result );
* // => 3
* }
*
* f( 6, done );
*/
function composeAsync() {
var nFuncs;
var f;
var i;
nFuncs = arguments.length;
if ( nFuncs < 2 ) {
throw new Error( 'insufficient arguments. Must provide multiple functions to compose.' );
}
f = new Array( nFuncs );
for ( i = 0; i < nFuncs; i++ ) {
f[ i ] = arguments[ i ];
if ( !isFunction( f[ i ] ) ) {
throw new TypeError( format( 'invalid argument. All arguments must be functions. Value: `%s`.', f[ i ] ) );
}
}
return composite;
/**
* Composite function.
*
* @private
* @param {...*} args - arguments
* @param {Callback} done - callback to invoke after invoking all functions
*/
function composite() {
var done;
var args;
var i;
// Cache the callback function:
done = arguments[ arguments.length-1 ];
// Copy arguments which should be provided to the first invoked function...
args = new Array( arguments.length-1 );
for ( i = 0; i < args.length; i++ ) {
args[ i ] = arguments[ i ];
}
// Append the callback an invoked function should call upon completion:
args.push( next );
// Start invoking provided functions:
i = nFuncs - 1;
f[ i ].apply( null, args );
/**
* Callback invoked upon completion of a composed function.
*
* @private
* @param {(Error|null)} error - error object
* @param {*} result - result to pass to next function
* @returns {void}
*/
function next( error, result ) {
if ( error ) {
return done( error );
}
i -= 1;
if ( i < 0 ) {
return done( null, result );
}
f[ i ]( result, next );
}
}
}
// EXPORTS //
module.exports = composeAsync;
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