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## Army of functions

importance: 5

The following code creates an array of `shooters`.

Every function is meant to output its number. But something is wrong…

``````function makeArmy() {
let shooters = [];

let i = 0;
while (i < 10) {
let shooter = function() { // shooter function
alert( i ); // should show its number
};
shooters.push(shooter);
i++;
}

return shooters;
}

let army = makeArmy();

army[0](); // the shooter number 0 shows 10
army[5](); // and number 5 also outputs 10...
// ... all shooters show 10 instead of their 0, 1, 2, 3...``````

Why all shooters show the same? Fix the code so that they work as intended.

Open a sandbox with tests.

Let’s examine what’s done inside `makeArmy`, and the solution will become obvious.

1. It creates an empty array `shooters`:

``let shooters = [];``
2. Fills it in the loop via `shooters.push(function...)`.

Every element is a function, so the resulting array looks like this:

``````shooters = [
function () { alert(i); },
function () { alert(i); },
function () { alert(i); },
function () { alert(i); },
function () { alert(i); },
function () { alert(i); },
function () { alert(i); },
function () { alert(i); },
function () { alert(i); },
function () { alert(i); }
];``````
3. The array is returned from the function.

Then, later, the call to `army[5]()` will get the element `army[5]` from the array (it will be a function) and call it.

Now why all such functions show the same?

That’s because there’s no local variable `i` inside `shooter` functions. When such a function is called, it takes `i` from its outer lexical environment.

What will be the value of `i`?

If we look at the source:

``````function makeArmy() {
...
let i = 0;
while (i < 10) {
let shooter = function() { // shooter function
alert( i ); // should show its number
};
...
}
...
}``````

…We can see that it lives in the lexical environment associated with the current `makeArmy()` run. But when `army[5]()` is called, `makeArmy` has already finished its job, and `i` has the last value: `10` (the end of `while`).

As a result, all `shooter` functions get from the outer lexical envrironment the same, last value `i=10`.

We can fix it by moving the variable definition into the loop:

``````function makeArmy() {

let shooters = [];

for(let i = 0; i < 10; i++) {
let shooter = function() { // shooter function
alert( i ); // should show its number
};
shooters.push(shooter);
}

return shooters;
}

let army = makeArmy();

army[0](); // 0
army[5](); // 5``````

Now it works correctly, because every time the code block in `for (let i=0...) {...}` is executed, a new Lexical Environment is created for it, with the corresponding variable `i`.

So, the value of `i` now lives a little bit closer. Not in `makeArmy()` Lexical Environment, but in the Lexical Environment that corresponds the current loop iteration. That’s why now it works.

Here we rewrote `while` into `for`.

Another trick could be possible, let’s see it for better understanding of the subject:

``````function makeArmy() {
let shooters = [];

let i = 0;
while (i < 10) {
let j = i;
let shooter = function() { // shooter function
alert( j ); // should show its number
};
shooters.push(shooter);
i++;
}

return shooters;
}

let army = makeArmy();

army[0](); // 0
army[5](); // 5``````

The `while` loop, just like `for`, makes a new Lexical Environment for each run. So here we make sure that it gets the right value for a `shooter`.

We copy `let j = i`. This makes a loop body local `j` and copies the value of `i` to it. Primitives are copied “by value”, so we actually get a complete independent copy of `i`, belonging to the current loop iteration.

Open the solution with tests in a sandbox.