[removed]What is meaning of sum(2)(3) //returns 5;

问题:

Here is code blow to return it's value.

function sum(a){
  return function(b){
    return a+b;
  }
}
sum(2)(3);

It returns 5 but if I type code:

function sum(a){
  function add(b){
    return a+b;
  }
return add(b);
}

It doesn't return expected value 5. I don't even understand how sum(2)(3) calls function. Any explanation for this is very much appreciated.

回答1:

This is called a closure.

sum(a) returns a function that takes one parameter, b, and adds it to a. Think of it like this:

   sum(2)(3);

   // Is equivalent to...
   function add(b){
       return 2+b;
   }
   add(3);

   // Which becomes...
   return 2+3; // 5

Your second snippet doesn't work because you're trying to reference b from the outer function, but only the inner function has any notion of what b is. You want to change this:

function sum(a){
  function add(b){
    return a+b;
  }
  return add(b);
}

To this:

function sum(a){
  function add(b){
    return a+b;
  }
  return add; // Return the function itself, not its return value.
}

Which is, of course, equivalent to the first snippet.

回答2:

Your 'sum' function returns another function which returns a+b (2 parameters). Both functions together require two parameters: (a) and (b)
The inner most return, returns a+b. Plugging in your parameters, we get the equation: 2+3.
Which gives you 5.

Please let me know if you have any questions or concerns.