Function Currying

What is currying

In one sentence: the process of turning a function that takes several arguments into a function that takes one argument and returns a function that accepts the rest is called currying a function. It is commonly used for: cutting down code volume when bundling JS scripts and shrinking the bundled file size; and for interview questions.

An example of currying — the classic add

Suppose we have this function:

function add(a, b) {
    return a + b
}

To curry it, we can rewrite it into the following shape:

function addCurry(a) {
    return function(b) {
        return a + b
    }
}

That is function currying.

An example of currying — add over a fixed number of numbers

Suppose we have this function:

function add(a, b, c) {
    return a + b + c
}

To curry it, we can rewrite it into the following shape:

function addCurry(a) {
    return function(b) {
        return function(c) {
            return a + b + c
        }
    }
}

If you think that's too redundant, you can try the following rewrite:

let addArray = []
function addCurry(...args) {
    addArray.push(...args)
    if (addArray.length > 3) {
        let result =  addArray.splice(0, 3).reduce((a, b) => {
            return a + b
        })
        addArray = []
        return result
    }
    return addCurry
}

An example of currying — add over a custom number of numbers

Suppose here we want a function that can add up a custom number of numbers; then we can rewrite the code from the previous question into the following shape:

function addInNumberOf(length) {
    let addArray = []
    return function addCurry(...args) {
        addArray.push(...args)
        if (addArray.length > length) {
            let result =  addArray.splice(0, length).reduce((a, b) => {
                return a + b
            })
            addArray = []
            return result
        }
        return addCurry
    }
}

This function is used like this:

let addThreeTimes = addInNumberOf(3);
console.log(addThreeTimes(1)(2)(3));  // prints 6

In this example:

• The first call, addInNumberOf(3), returns a new function, which we assign to the variable addThreeTimes.
• By calling addThreeTimes(1)(2)(3) in succession we get the result 6.
• Because every call pushes the new argument into the array, once the array reaches length 3 it computes 1+2+3 and returns the result.

Important

This article was translated by Hermes Agent; the original text can be found at here. The translation is for reference only. Translation time: 2026-11-05