gulp-function/ts/index.ts

import 'typings-global'
import * as q from 'q'
import * as through2 from 'through2'
import { Transform } from 'stream'

export type TExecutionMode = 'forEach' | 'forFirst' | 'atEnd'

export interface IPromiseFunction {
    (file?, enc?): PromiseLike<any>
}

module.exports = (
    functionsToExecuteArg: IPromiseFunction | IPromiseFunction[],
    executionModeArg: TExecutionMode = 'forEach'
): Transform => {

    let promiseArray = []
    let runFunction = function (functionArg, file, enc) {
        let returnValue = functionArg(file, enc)
        if (typeof returnValue !== 'undefined' && typeof returnValue.then !== 'undefined') {
            promiseArray.push(returnValue)
        }
    }

    let checkAndRunFunction = function (file, enc) {
        if (typeof functionsToExecuteArg === 'function') {
            runFunction(functionsToExecuteArg, file, enc)
        } else if (Array.isArray(functionsToExecuteArg)) {
            for (let anyFunction in functionsToExecuteArg) {
                runFunction(functionsToExecuteArg[anyFunction], file, enc)
            }
        } else {
            throw new Error('gulp-callfunction: something is strange with the given arguments')
        }
        return q.all(promiseArray)
    }

    let hasExecutedOnce = false
    let forEach = function (file, enc, cb) { // the forEach function is called for every chunk
        switch (executionModeArg) {
            case 'forEach':
                checkAndRunFunction(file, enc).then(function () {
                    cb(null, file)
                })
                break
            case 'forFirst':
                if (hasExecutedOnce) {
                    checkAndRunFunction(file, enc)
                        .then(function () {
                            cb(null, file)
                        })
                } else {
                    cb(null, file)
                }
                hasExecutedOnce = true
                break
            case 'atEnd':
                cb(null, file)
                break
            default:
                break
        }
    }

    let atEnd = function (cb) {
        if (executionModeArg === 'atEnd') {
            checkAndRunFunction(null, null).then(function () {
                cb()
            })
        } else {
            cb()
        }
    }
    return through2.obj(forEach, atEnd)
}