CODEWITHSUNDEEP
javascript
user3730820
user3730820

Reputation:

Javascript toString trick in adding function. How does it work?

Can somebody explain to me how the toString trick works in the following example. The function adds all the arguments being passed for each call so add(1)(2)(3); equals 6.

jsFiddle

function add(a) {
  var sum = a;
    return function self(a) {
      sum += a;
      self.toString = function () {
          return sum;
      }
      return self;
  }
}

console.log(add(1)(2)(3));

Upvotes: 4

Views: 121

Answers (3)

Daniel A. White
Daniel A. White

Reputation: 190915

It is returning itself which is a function. The self function also closes around sum. Then console.log is using the overridden toString.

Basically, this is happening

var a = add(1); //a is a self
var b = a(2); //b is the same self (b === a)
var c = b(3); //c is a same self (c === a && c === b)
console.log(c); // calls c.toString()

It isn't the add function getting called multiple times, it's the self function.

Upvotes: 1

Guffa
Guffa

Reputation: 700252

The function add declares a variable sum and returns a function that has two purposes:

  1. You can call the function with a value, which will add the value to sum and return the same function.

  2. You can get the string value of the function, which will return the value of sum.

So, the value of add(1) is a function, where sum is 1.

The value of add(1)(2) is a function, where sum is 3.

The value of add(1)(2)(3) is a function, where sum is 6.

The console.log call will get the string value of the function, which is sum.

You can also write basically the same code like this, which might be somewhat easier to follow:

function add(a) {
    var sum = a;
    function self(a) {
        sum += a;
        return self;
    }
    self.toString = function () {
        return sum;
    };
    return self;
}

console.log(add(1)(2)(3));

(The difference is that toString is bound by the function add, not each time that you call the returned function. The edge case is that console.log(add(1)) shows 1 instead of self(a).)

Upvotes: 1

Niet the Dark Absol
Niet the Dark Absol

Reputation: 324620

Since the function is a chain function, you need to return a function to be chained which then returns itself.

However, you also need a meaningful way to get its result.

In jQuery, for example, you might see .get() used to extract results from a chained operation. This is much the same, using .toString() to mean "if you're putting me somewhere a string is expected, return the result".

Upvotes: 3

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