up

1-js/06-advanced-functions/03-closure/8-make-army/solution.md

@@ -0,0 +1,121 @@
+
+Let's examine what's done inside `makeArmy`, and the solution will become obvious.
+
+1. It creates an empty array `shooters`:
+
+    ```js
+    let shooters = [];
+    ```
+2. Fills it in the loop via `shooters.push(function...)`.
+
+    Every element is a function, so the resulting array looks like this:
+
+    ```js no-beautify
+    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:
+
+```js
+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`.
+
+The fix can be very simple:
+
+```js run
+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 (..) {...}` is executed, a new Lexical Environment is created for it, with the corresponding value of `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. A `shooter` gets the value exactly from the one where it was created.
+
+![](lexenv-makearmy.png)
+
+Here we rewrote `while` into `for`.
+
+Another trick could be possible, let's see it for better understanding of the subject:
+
+
+```js run
+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.