Javascript closure scope confusion

Question

I am confused with the scoping of closure expression. I don't understand why output is 40 only.

js

  function multiply(input) {
      var no = 5;
      function multiply2(mul) {
        mul *= input;
        return no * mul;
      }
      return multiply2;
    }
    
    var total = multiply(4);
    var result = total(2);
console.log("result :",result);

Output

result :40

What is the expected value? What exactly is the problem? What exactly don’t you understand? — Sebastian Simon
– Sebastian Simon, Commented Jun 15, 2016 at 1:53
But you wrote it at the beginning of the question: 2 * 4 = 8 and 8 * 5 = 40. — Felix Kling
– Felix Kling, Commented Jun 15, 2016 at 1:58
I knew the answer so I assumed that like something like formula is happening.But I don't know why? — Dhwani sanghvi
– Dhwani sanghvi, Commented Jun 15, 2016 at 2:01
Because mul *= input; and return no * mul;. Now you need to look at what the values of mul, input and no are. — Felix Kling
– Felix Kling, Commented Jun 15, 2016 at 2:06

Paul · Accepted Answer · 2016-06-15 02:06:28Z

1

multiply(4) returns a function multiply2 which has two free variables in scope: no which has the value 5, and input which has the value 4. This line assigns that returned function to total:

var total = multiply(4);

total(2) calls that function, passing in 2 for the argument mul. The definition of that function is two statements, but it could be simplified to one statement (since mul is a local variable and is never used after modifying it, that side-effect can be safely dropped): return no*mul*input;. mul was passed in, with the value 2; no and input are the free variables that were created when multiply(4) was called and we know what they are set to, so, in the statement below, result becomes 5*2*4, which is 40.

var result = total(2);

edited Jun 15, 2016 at 2:06

answered Jun 15, 2016 at 1:57

Paul

142k28 gold badges285 silver badges272 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Dhwani sanghvi Over a year ago

Thanks for the explaination.It helps me to understand the concept.

Enjayy · Accepted Answer · 2016-06-15 01:59:27Z

0

The output 40 is correct. Your functions multiply 4*2 = 8 and then you multiply 8*5 = 40. I think if you aren't getting what you expect is because you are multiplying 4*2 and making mul = 8 so what then gets returned is 5*8

answered Jun 15, 2016 at 1:59

Enjayy

1,0849 silver badges18 bronze badges

Comments

Mengo · Accepted Answer · 2016-06-15 02:37:12Z

0

This is functional programming called currying. An abstract implementation would be y = f(m) => g(n) => m*n. So your input to f will be carried into g and return a function which is g(n). Then your input to g will return the final result.

This can be implemented more than one level. ie. y = f(m)=>g(n)=>h(p)=>m*n*p, where the input value will be carried to next function until the final input.

Here is some basics of functional programming. Also a great book here.

edited Jun 15, 2016 at 2:37

answered Jun 15, 2016 at 2:18

Mengo

1,2971 gold badge12 silver badges29 bronze badges

Collectives™ on Stack Overflow

Javascript closure scope confusion

js

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

js

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related