How to implement currying in javascript?

Question

Question as below:

create a sum function, and the requirement:

sum(1,2).result === 3
sum(1,2)(3).result == 6
sum(1,2)(3,4).result == 10
sum(1,2)(3,4)(5).result == 15

This is a question about currying in JS. I have implemented the most functions of the question. The tricky point is .result for me.

What does .result means after sum(1,2)? Is it an attribute?

How to add the .result to my code?

function sum(){
    var count = 0;
    for(let i=0; i<arguments.length; i++){
        count += arguments[i];
    }
    var tmp = function(){
        for(let i=0; i<arguments.length; i++){
            count += arguments[i];
        }
        return tmp;
    }
    tmp.toString = function(){
        return count;
    }
    return tmp;
}

console.log(sum(1,2))
console.log(sum(1,2)(3))

Yes, `result` would be an property. Functions in JS are still objects, so they can have properties or even *methods* attached to them. — VLAZ, Dec 10 '19 at 12:44

Shrey Gupta · Answer 1 · 2019-12-10T15:28:05.997

3

You can add .result to your code by storing result as a property in function.

You can also remove the redundant tmp function code . Here is how it looks:

function sum(){
    var count = 0;    // use previous result
    for(let i=0; i<arguments.length; i++){
        count += arguments[i];
    }
    const newSum = sum.bind(this,count);
    newSum.result = count;
    return newSum;
}

console.log(sum(1,2).result)
console.log(sum(1,2)(3).result)

.bind() here binds the first argument of cloned function (newSum) as count , which will be used on all subsequent calls

edited Dec 10 '19 at 15:28

answered Dec 10 '19 at 14:39

Shrey Gupta

392
4
10

Its always good to mark answer as correct . If the solution fits your needs – Shrey Gupta Dec 11 '19 at 15:25

score 0 · Answer 2 · answered Jul 23 '20 at 17:43

Shrey answer is Perfect but we can make it more sorter and clear.

function sum() {
    let result = [...arguments].reduce((acc, num) => acc + num, 0);
    let newSum = sum.bind(this, result);
    newSum.result = result;
    return newSum;
}

console.log(sum(3,4)(5)(9).result);
console.log(sum(3,4)(5,6)(9).result);

score 0 · Answer 3 · answered Aug 05 '20 at 06:13

Currying is an advanced technique of working with functions. It’s used not only in JavaScript, but in other languages as well.

Currying is a transformation of functions that translates a function from callable as f(a, b, c) into callable as f(a)(b)(c).

Currying doesn’t call a function. It just transforms it.

Let’s see an example first, to better understand what we’re talking about, and then practical applications.

We’ll create a helper function curry(f) that performs currying for a two-argument f. In other words, curry(f) for two-argument f(a, b) translates it into a function that runs as f(a)(b):

function curry(f) { // curry(f) does the currying transform
  return function(a) {
    return function(b) {
      return f(a, b);
    };
  };
}

// usage
function sum(a, b) {
  return a + b;
}

let curriedSum = curry(sum);

alert( curriedSum(1)(2) ); // 3

Also Refer this link for more details

score 0 · Answer 4 · answered Aug 05 '20 at 06:21

Here a solution. It always better to use spread operator on the arguments instead of using arguments keyword. Witch should be avoided

function sum(...args){
    let count = 0;
    count += args.reduce((a,v) => a + v);
    let res = sum.bind(this,count);
    res.result = count;
    return res;
}

console.log(sum(1,5)(4)(5).result)

How to implement currying in javascript?

4 Answers4