JavaScript closure parameters

Question

foo is equal to iCantThinkOfAName, but the iCantThinkOfAName has two parameters, and foo only one. I don’t understand how foo(2) returns num: 4.

function iCantThinkOfAName(num, obj) {
  // This array variable, along with the 2 parameters passed in, 
  // are 'captured' by the nested function 'doSomething'
  var array = [1, 2, 3];

  function doSomething(i) {
    num += i;
    array.push(num);
    console.log('num: ' + num);
    console.log('array: ' + array);
    console.log('obj.value: ' + obj.value);
  }
  return doSomething;
}
var referenceObject = {
  value: 10
};
var foo = iCantThinkOfAName(2, referenceObject); // closure #1
var bar = iCantThinkOfAName(6, referenceObject); // closure #2
foo(2);

/*
num: 4
array: 1,2,3,4
obj.value: 10
*/

what is num with foo(3)? 5 - what is num with bar(2) - 8 ... does that help? — Jaromanda X
– Jaromanda X, Commented Feb 17, 2017 at 3:58
Why don’t you understand it? It’s just how closures work. What did you expect instead? — Sebastian Simon
– Sebastian Simon, Commented Feb 17, 2017 at 4:01

rgthree · Accepted Answer · 2017-02-17 04:07:26Z

3

foo is not equal to iCantThinkOfAName, it is equal to iCantThinkOfAName(2, referenceObject) which returns the inner function doSomething within a closure containing num (which is equal to the 2 you passed in), obj which is your referenceObject you passed in, and array = [1, 2, 3];.

When you call foo(2) you are directly calling that inner doSomething where i is your 2 you are passing in, which gets added to your num which was your original 2; thus, num within the closure, is now 4.

answered Feb 17, 2017 at 4:07

rgthree

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1 Comment

Sebastian Simon Over a year ago

This is actually a really clear and great explanation of what’s going on in the code and what each function does.

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