javascript中的cururing功能

Question

Can someone please explain the below 'Currying' function. When I set a break point, I can see values 'b' and 'c' are undefined and returned. But how does the system get the value 7 for 'b' and 9 for 'c' in this case. Its printing the correct result for 5*7*9, ie 315 in console.

function multiply(a){
    return function (b){
        return function (c) {       
            return a*b*c ;
                }
        }
    }
var answer = multiply(5)(7)(9);
console.log(answer);

Answer 1

When you call multiply(5) a function is returned and that function is immediately called with multiply(5)(7) and have access to a and b , then a new function is returned and is also immediately called when multiply(5)(7)(9) and has access to a , b , and c .

The answer to why the nested functions have access to parent function parameters is closures . Please check this other question/answers about closures in javascript: link .

Answer 2

You can understand currying if you assign each intermediate result to a variable. Rewrite the function calls like this:

var x = multiply(5)
var y = x(7);
var answer = y(9);

Now it is more clear that the result of calling multiply(5) is a function. We call it with x(7) which passes the value of 7 to b . This results in another function which we call with y(9) which in turn passes the value of 9 to c and returns the final result.

Answer 3

When you call the multiply function, the important thing is each function returns an anonymous function. It will be easier if you give them names.

function multiplyA(a){
    return function multiplyB(b){
        return function multiplyC(c) {       
            return a*b*c ;
        }
    }
}

// When you call multiplyA(5)(7)(9)
// From the first, multiplyA(5) returns multiplyB with scope with multiplyA
// just like below

const a = 5;
const multiplyB = function(b){
    return function multiplyC(c) {       
        return a*b*c ;
    }
}

// Once more call multiplyB(7)
// It also returns function named multiplyC

const a = 5;
const b = 7;
const multiplyC = function(c) {
    return a * b * c;
}

// Finally call mulitplyC(9) means 5 * 7 * 9

To understand it, closure and function scope will be helpful.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures