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);
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.
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.
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
a*b*cis run, the parametersa,bandcwill all have numeric values in this case. And I don't understand what you mean by the "how system understand and need to grab..." part.bis just a parameter of the function. The fact that the functions are nested doesn't change that.console.log(a, b, c)before the multiplication line.const multiply = (a) => (b) => (c) => a*b*c;) made it a lot clearer to me what I was doing.