Javascript array with callback function

Question

I tried to have better understanding of JavaScript. Here is a piece of code that I read from JavaScript function closures.

var funcs = [];
// create a bunch of functions
for (var i = 0; i < 3; i++) {
   funcs.push(function() {
    console.log(i);
   })
}
// call them
for (var j = 0; j < 3; j++) {
  funcs[j]();
}

The array funcs has a push callback function. I don't why in the J loop, funcs[j]() will call this function to print the i in the console.
I have tried to understand this sequencey by adding some console messages:

var funcs = [];
console.log("start");
for (var i = 0; i < 3; i++) {
  console.log("i:" + i);
  funcs.push(function(){
    console.log(i);
  })
}

console.log("J loop");
for (var j=0; j<3; j++) {
  console.log("j:" + j);
  funcs[j]();
}

As expected, there is 3 for all three functions.
My question is: How does funcs[j]() calls the funcs.push(...) function? I understant the funcs[j] is reference the j element of the funcs array. But why having parentheses () will call the push(...) function?

It doesn't call the `.push()` function; it calls the function that was *passed* to `.push()`, the little function with `console.log(i);` inside. — Pointy, Mar 21 '16 at 15:40
@Pointy can you explain a little bit more? That's what I am confused about. Why does it call the function that was passed to `.push()`? It is a syntax or something else? — Shaohao, Mar 21 '16 at 15:41
.push add element at the end of the array. In first loop you pushed a function to the array. In second loop , you are invoking the function with () [] indicating index. — Darlyn, Mar 21 '16 at 15:42
Possibly separately from what confusing you, the code above also has the issue that the value of `i` may surprise you: http://stackoverflow.com/questions/750486/javascript-closure-inside-loops-simple-practical-example — T.J. Crowder, Mar 21 '16 at 15:44

score 11 · Accepted Answer · answered Mar 21 '16 at 15:42

11

function() {console.log(i);} is an expression which evaluates to a value that is function that logs i.

funcs.push is a function that adds a value to an array.

Putting () after a function will call that function.

funcs.push(some_value) calls the push function and passes some_value as the value to put in the array.

funcs.push(function() {console.log(i);}) adds the function to the array.

The value of funcs[0] becomes that function.

Putting () after a function will call that function.

funcs[0]() calls the function that is the first value in the array.

answered Mar 21 '16 at 15:42

Quentin

914,110
126
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1

Thank you for the step by step explanation! It's very helpful! – Shaohao Mar 21 '16 at 15:48
So putting `()` after a function will always invoke whatever function in the `push()`? – Shaohao Mar 21 '16 at 15:55
1

Putting `()` after a function will invoke that function. Putting it after something which represents the function which was pushed into the array will invoke that function. – Quentin Mar 21 '16 at 15:56

score -1 · Answer 2 · answered Mar 21 '16 at 15:50

-1

First, the 'i' variable are global, and ending the loop, i=3 Then, the functions inside funcs, use the variable "i", then, all functions print "3" in console.

Maybe you wanted to do this:

var funcs = [];
console.log("start");
for (var i = 0; i < 3; i++) {
  console.log("i:" + i);
  funcs.push(function(i){
    console.log(i);
  })
}

console.log("J loop");
for (var j=0; j<3; j++) {
  console.log("j:" + j);
  funcs[j](j);
}

answered Mar 21 '16 at 15:50

Arnau Castellví

240
2
11

1

Thanks for the answer, but I don't have a problem to understand why it prints `3` three times. I just have a problem about why `()` will invoke the `console.log(i)` function. And @Quentin did a great job to explain it. – Shaohao Mar 21 '16 at 15:52

Javascript array with callback function

2 Answers2