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How to create a composition from functions

I have 5 functions: func1(), func2(), func3(), func4(), func5(). I need to implement the compositionFunc() function, which can take any number of functions as arguments, and create a composition from them. The compositionFunc() function takes my 5 functions as arguments. The compositionFunc() function returns a function that takes its initial value as an argument. This nested function successively passing through an array of functions with each iteration returns the result of calling the accumulated value of the current function-argument. The result of one function can be passed as an argument to another function. How can i do this?
const func1 = (arg1) => {
return arg1;
};
const func2 = (arg2) => {
return arg2;
};
const func3 = (arg3) => {
return arg3;
};
const func4 = (arg4) => {
return arg4;
};
const func5 = (arg5) => {
return arg5;
};
const compositionFunc = () => {
...
};

you can define a function like this
const pipe = (...functions) => args => functions.reduce((res, f) => f(res), args)
const combine = (...functions) => args => functions.reduceRight((res, f) => f(res), args)
const plus1 = x => x + 1
const double = x => x * 2
const pipeFunction = pipe(plus1, double)
const combineFunction = combine(plus1, double)
console.log(combineFunction(1)) // (1 * 2) + 1
console.log(pipeFunction(1)) // (1 + 1) * 2

A simple reduce can accomplish that:
function pipe(input, ...func) {
return func.reduce((a, f) => f(a), input);
}
You pass it an initial value + chain of functions.
Example:
function f1(val) {
return val + 1;
}
function f2(val) {
return val * 10;
}
console.log(pipe(2, f1, f2)); //=> 30

How to run multiple async functions as fast as possible (JS)?

If you were given an array of async functions and the task is to create a class that takes this array and runs the functions as fast as possible with a constraint that only 15 functions can run at the same time, what would be a way to do that?
If there wasn't a constraint for 15 functions, I believe Promise.all would be the way to go.
Using just async/await and waiting for one function to resolve to add the next one is very slow as we must have to wait for 1 function to resolve until we can add another one and we can thus have a bottleneck function.
Adding 15 functions to array and running them with Promise.all and after that resolves, adding another 15 or the rest of them, is again, not very efficient as what we want to do is to call another function as soon as one of the functions resolves.
Any ideas?

Let's create a stack that has an async popAsync method:
const createAsyncStack = () => {
const stack = [];
const waitingConsumers = [];
const push = (v) => {
if (waitingConsumers.length > 0) {
const resolver = waitingConsumers.shift();
resolver && resolver(v);
}
else {
stack.push(v);
}
};
const popAsync = () => {
if (stack.length > 0) {
const queueItem = stack.pop();
return typeof queueItem !== 'undefined'
? Promise.resolve(queueItem)
: Promise.reject(Error('unexpected'));
}
else {
return new Promise((resolve) => waitingConsumers.push(resolve));
}
};
return [push, popAsync];
};
This means that any consumer calling popAsync will be returned a Promise that only completes if / when an item is available in the stack.
We can now use this stack as a "gatekeeper" for a simple higher-order function (i.e. a function that returns a function).
Say we only want to allow maxDOP (maximum degrees-of-parallelism) concurrent invocations of an async function, we push maxDOP tokens into the stack (here, I've used empty objects as the tokens), then require that in order to proceed, it is necessary to acquire a token from this stack. When our function call is finished, we return our token to the stack (using push), where that token can then be consumed by any waiting consumers.
const withMaxDOP = (f, maxDop) => {
const [push, popAsync] = createAsyncStack();
for (let x = 0; x < maxDop; ++x) {
push({});
}
return async (...args) => {
const token = await popAsync();
try {
return await f(...args);
}
finally {
push(token);
}
};
};
The function returns a new function that can be called in exactly the same way as the function that is supplied to it (i.e. is has the same signature).
Now, let's create a function that simply calls a supplied function with the supplied arguments:
const runAsync = (asyncFn, ...args) => asyncFn(...args);
and wrap it using the higher-order withMaxDOP function, which will return a new function with an identical signature to the wrapped function:
const limitedRunAsync = withMaxDOP(runAsync, 15);
Now we can use this function to call the functions in our array:
Promise.all(asyncFns.map(f => limitedRunAsync(f)))
.then((returnValues) => console.log("all finished", returnValues));
which will ensure that there are only ever 15 "in-flight" invocations ever permitted at one time.
See this runnable snippet for a full example:
const createAsyncStack = () => {
const stack = [];
const waitingConsumers = [];
const push = (v) => {
if (waitingConsumers.length > 0) {
const resolver = waitingConsumers.shift();
resolver && resolver(v);
} else {
stack.push(v);
}
};
const popAsync = () => {
if (stack.length > 0) {
const queueItem = stack.pop();
return typeof queueItem !== 'undefined' ? Promise.resolve(queueItem) : Promise.reject(Error('unexpected'));
} else {
return new Promise((resolve) => waitingConsumers.push(resolve));
}
};
return [push, popAsync];
};
const withMaxDOP = (f, maxDop) => {
const [push, popAsync] = createAsyncStack();
for (let x = 0; x < maxDop; ++x) {
push({});
}
return async(...args) => {
const token = await popAsync();
try {
return await f(...args);
} finally {
push(token);
}
};
};
const runAsync = (asyncFn, ...args) => asyncFn(...args);
const limitedRunAsync = withMaxDOP(runAsync, 15);
// set up an array of async functions
const delay = (durationMS) => new Promise((resolve) => setTimeout(() => resolve(), durationMS));
const asyncFns = [...Array(50)].map((_, i) => () => {
console.log("starting " + i);
return delay(Math.random() * 5000).then(v => {
console.log("finished " + i);
return i;
});
});
// ...then wrap and call them all at once
Promise.all(asyncFns.map(f => limitedRunAsync(f))).then((returnValues) => console.log("all finished", returnValues));
...and see this TypeScript Playground Link for a fully type-annotated version of the same code.

Here's something I whipped up in the last 20 minutes that should do the job
I'm sure if I thought about it I could probably do it without the Promise constructor, but ... 20 minutes is 20 minutes :p
Please, if someone can rewrite this without the Promise constructor, I'd love to see it - because in the back of my mind, I'm sure there is a way
Note, this will run regardless of rejections
Results will be either
result: actualResult
or
error: rejectionReason
So you can process results/rejections
function runPromises(arrayOfFunctions, maxLength) {
return new Promise(resolve => {
const queue = arrayOfFunctions.map((fn, index) => ({fn, index}));
const results = new Array(arrayOfFunctions.length);
let finished = 0;
const doQ = () => {
++finished;
if (queue.length) {
const {fn, index} = queue.shift();
fn()
.then(result => results[index] = {result})
.catch(error => results[index] = {error})
.finally(doQ);
} else {
if (finished === arrayOfFunctions.length) {
resolve(results);
}
}
};
queue.splice(0, maxLength).forEach(({fn, index}) => fn()
.then(result => results[index] = {result})
.catch(error => results[index] = {error})
.finally(doQ)
);
});
}
//
// demo and show that maximum 15 inflight requests
//
let inFlight = 0;
let maxInFlight = 0;
const fns = Array.from({length:50}, (_, i) => {
return () => new Promise(resolve => {
++inFlight;
maxInFlight = Math.max(inFlight, maxInFlight);
setTimeout(() => {
--inFlight;
resolve(i);
}, Math.random() * 200 + 100,)
});
});
runPromises(fns, 15).then(results => console.log(maxInFlight, JSON.stringify(results)));

Executing list of async functions

I have an exercise to make a function executeFunctions which takes as arguments a list of async functions and an argument, e.g. number.
The functions have to happen one after another, so if fun1 ends, fun2 needs to start with the value which was returned from fun1.
The problem is that I can't use async and await. I wanted to do it using reduce, but I guess that it wants to execute const res1 and go further before it returns a value (because of setTimeout).
Is there any way to do it without async and await?
const fun1 = function(value) {
return setTimeout(() => value*2, 3000)
}
const fun2 = function(value) {
return setTimeout(() => value*4, 3000)
}
const cb2 = (value) => {
return value*10
}
const executeFunctions = (funTab, cb) => (n) => {
const res1= funTab[0](n)
console.log(res1)
const resReduce = funTab.reduce((prev,curr) => {
const res2 = curr(prev)
return prev+res2
}, res1)
return cb(resReduce)
};
executeFunctions([fun1,fun2], cb2)(2)

We can use Promise-chaining:
const fun1 = function(value) {
return Promise.resolve(value * 2);
}
const fun2 = function(value) {
return Promise.resolve(value * 2);
}
const fun3 = function(value) {
return Promise.resolve(value * 2);
}
const executeFunctions = (funcList) => (n) => {
let chain = Promise.resolve(n); // initial value
for (let i = 0; i < funcList.length; i++) {
chain = chain.then(funcList[i]); // keep chaining
}
return chain; // last promise
};
const main = () => {
// we need to wait for the last promise in order to print the result
executeFunctions([fun1, fun2, fun3])(2).then(x => console.log('solution is:', x));
}
main() // prints: "solution is: 16"
or, we can also use a modified version of the suggested reduce solution, by changing the implementation of executeFunctions as follows (the rest of the code should remain as in the previous snippet):
const executeFunctions = (funcList) => (n) => {
const init = Promise.resolve(n);
const res = funcList.reduce((p, c) => {
return p.then(c)
}, init);
return res;
};

passing argument to map function

I have code like:
var addX = (e) => return e+1;
I am calling like:
[1,2,3].map(addX);
Can I make "1" dynamic? Like:
[1,2,3].map(addX(2)); //this wont work, in this case it should add 2

You need a closure over e and return a function with one parameter for the callback.
var addX = x => v => x + v;
console.log([1, 2, 3].map(addX(2)));

You can use currying like this:
var addX = n => e => e + n;
console.log([1,2,3].map(addX(2)));
console.log([1,2,3].map(addX(10)));
What this does is, you pass the number (n) that you want to be added to the function and it returns a new function which adds that number to it's argument (e) (each element when using map)

You can use bind(thisArg, argument1) to send an extra parameter
var addX = (a, b) => a+b;
console.log("2", [1,2,3].map(addX.bind(Array, 2)))
console.log("5", [1,2,3].map(addX.bind(Array, 5)))
The this argument really does not matter in this case. I just put Array there, it can be this, null, etc.

You can create a function to return a function that takes your add amount.
Here is an example below.
var addX = (addbit) => (e) => e + addbit;
console.log([1,2,3].map(addX(2)));

I would do something like this :)
var addX = (e) => e+1;
[1,2,3].map(num => addX(num))

Several utility libraries like Lodash, Underscore, and Ramda provide a curry() utility function that takes a function and returns a new version of that function that can take the same arguments one at a time, a few at a time, or all at once.
This means that you can create an add function that can either add two numbers right away add(4, 5) or that can "pre-bake" a function with one of the values included: const increment = add(1); console.log(increment(5));
const add = _.curry((x, y) => x + y);
console.log(add(6, 7));
console.log([1,2,3].map(add(10)));
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.4/lodash.min.js"></script>
If you don't want to include a whole library just for this purpose, you can also use this standalone version of curry:
var curryN = (function () {
var slice = Function.prototype.call.bind(Array.prototype.slice);
var bindArr = function (f, arr) { return f.bind.apply(f, [{}].concat(arr)); };
return function curryN(argCount, func) {
return function fn() {
var args = slice(arguments, 0, argCount);
return args.length === argCount
? func.apply({}, args)
: bindArr(fn, args);
};
};
})();
var curry = function curry(func) { return curryN(func.length, func); };
const add = curry((x, y) => x + y);
console.log(add(6, 7));
console.log([1,2,3].map(add(10)));

You can use closure for that:
var addX = inc => e => e + inc;
var res = [1,2,3].map(addX(2));
console.log(res);
EXPLANATION
var addX = inc => e => e + inc;
Is the equivalent of:
var addX = function(inc) {
return function(e) {
return e + inc;
}
}
So addX(2) returns the callback function(e) { return e + 2 } and it's the callback used by array.map

piping functions in JavaScript

How can I have a JavaScript function let's say piper() which takes several functions as its arguments and it returns a new function that will pass its argument to the first function, then pass the result to the second, then
pass the result of the second to the third, and so on, finally returning the output of the last function.
Something like piper(foo, fee, faa)(10, 20, 30) would be equivalent to calling faa(fee(foo(10,20,30))).
ps:
It was a part of an interview, that I did few days ago.

For an arbritrary number of functions you could use this ES6 function:
function piper(...fs) {
return (...args) => fs.reduce((args,f) => [f.apply(this,args)],args)[0];
}
// Example call:
var result = piper(Math.min, Math.abs, Math.sqrt)(16, -9, 0)
// Output result:
console.log(result);
The same in ES5 syntax:
function piper(/* functions */) {
var fs = [].slice.apply(arguments);
return function (/* arguments */) {
return fs.reduce(function (args,f) {
return [f.apply(this,args)];
}.bind(this), [].slice.apply(arguments))[0];
}.bind(this);
}
// Example call:
var result = piper(Math.min, Math.abs, Math.sqrt)(16, -9, 0)
// Output result:
console.log(result);

Enjoy. Pure ES5 solution. Preserves this.
function piper(){
var i = arguments.length,
piped = arguments[ --i ];
while( --i >= 0 ){
piped = pipeTwo( arguments[ i ], piped );
}
return piped;
}
function pipeTwo( a, b ){
return function(){
return a.call( this, b.apply( this, arguments ) );
}
}
Or, if you want the fancy solution.
function piperES6( ...args ){
return args.reverse().reduce( pipeTwo );
}
Loops can be reversed depending on the desired direction.

Very similar to #trincot's answer (preserves context), but composes in the correct order and is marginally faster since it does not create intermediary arrays:
const piper = (...steps) => function(...arguments) {
let value = steps[0].apply(this, arguments);
for (let i = 1; i < steps.length; ++i) {
value = steps[i].call(this, value);
}
return value;
};
// Usage:
let p = piper(
x => x + 1,
x => x * 2,
x => x - 1
);
console.log(p(2)); // 5

Here is an alternative answer involving method chaining. I shall use ES6, though of course this can be transpiled to ES5. On benefit of this solution is that is has a very succinct TypeScript counterpart with perfect typeability.
class Pipe {
constructor(value) {
this.value = value;
}
then(f) {
return new Pipe(f(this.value));
}
}
const pipe = value => new Pipe(value);
// Example
const double = x => 2 * x;
pipe(42).then(double).then(console.log); // 84
const result = pipe(42).then(double).then(double).value;
console.log(result); // 168

A simple solution based on JS higher-order functions usage:
function pipe(...rest) {
return x => rest.reduce((y, f) => f(y), x);
}
Usage:
pipe((a) => a + 1, (a) => a * 2)(3) // 8
pipe((a) => a + 1, (a) => a * 2)(2) // 2

function f(f1, f2, f3){
return (args => f3(f2(f1(args))));
}

I think what you are trying to do is chaining.
var funct={
total:0,
add:function(a) {
console.log(funct.total,funct.total+a);
funct.total+=a;
return funct;
}
};
funct.add(5).add(6).add(9);