Javascript variables-- What does it mean when add () after the variable?

Question

I just started learning js from EloquentJavascript I need 2 answers to help me pass this example:

function makeAddFunction(amount) {
  function add(number) {
    return number + amount;
  }
  return add;
}

var addTwo = makeAddFunction(2);
var addFive = makeAddFunction(5);
show(addTwo(1) + addFive(1));

Question 1:

Can we just add () after variable meaning to add argument to the function that the variable represent? like addTwo(1) means makeAddFunction(1)?

Question 2:

makeAddFunction(2) means amount = 2, then what's the value for number? If I pick a section of the above example, what will return in the following section?

function makeAddFunction(2) {
  function add(number) {
    return number + 2;
  }
  return add;
}

the first example returns 9, the 2nd example returns ReferenceError: amount is not defined. — Jenny
– Jenny, Commented Jun 9, 2012 at 22:25
You're doing something wrong. show us a fiddle. And the question is too localized, You're asking a question about a concrete code... — gdoron
– gdoron, Commented Jun 9, 2012 at 22:27

zerkms · Accepted Answer · 2012-06-09 22:26:41Z

2

Can we just add () after variable meaning to add argument to the function that the variable represent? like addTwo(1) means makeAddFunction(1)

Nope. addTwo(1) equals to makeAddFunction(2)(1)

makeAddFunction(2) means amount = 2, then what's the value for number? in the following example2, what will return?

You pass it additionally, when call addTwo or addFive

answered Jun 9, 2012 at 22:26

zerkms

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5 Comments

Jenny Over a year ago

So, when there's additional var inside a function, we can pass additional value like makeAddFunction(2)(1) ?

zerkms Over a year ago

@Jenny: it's not additional var. makeAddFunction returns a function, that accepts one argument. So result of makeAddFunction(1) call is another function, that accepts one argument

Jenny Over a year ago

I see. so, in this case, I need to give argument to the another function by adding the argument to the var addTwo and addFive?

zerkms Over a year ago

@Jenny: yes, because they are functions

Jenny Over a year ago

Thanks! This helps get my head straight.

Sascha Galley · Accepted Answer · 2012-06-09 22:25:03Z

2

Both of your questions are answered if you look at the return type of makeAddFunction(amount). You are returning a function add(number). So by defining addTwo as makeAddFunction(2), you are returning the following function:

function add(number) {
    return number+2;
}

If addTwo and addFive are function, show(...) will show: 9. (2+1)+(5+1)=9

answered Jun 9, 2012 at 22:25

Sascha Galley

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2 Comments

Jenny Over a year ago

addTwo and addFive are not function. they are variables defined just below the function makeAddFunction. That's why I doubt if I can pass argument through variable by adding () after variable.

zerkms Over a year ago

@Jenny: they are functions. You can store function in a variable. var foo = function() { return '123'; }; alert(foo()); --- this will alert 123

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