diff --git a/1-js/06-advanced-functions/04-var/article.md b/1-js/06-advanced-functions/04-var/article.md index 1e8bb5ba..b2559989 100644 --- a/1-js/06-advanced-functions/04-var/article.md +++ b/1-js/06-advanced-functions/04-var/article.md @@ -28,7 +28,7 @@ On the other hand, it's important to understand differences when migrating old s ## "var" has no block scope -Variables, declared with `var`, are either function-wide or global. They are visible through blocks. +Variables, declared with `var`, are either function-scoped or global-scoped. They are visible through blocks. For instance: @@ -60,11 +60,13 @@ The same thing for loops: `var` cannot be block- or loop-local: ```js for (var i = 0; i < 10; i++) { + var one = 1; // ... } *!* -alert(i); // 10, "i" is visible after loop, it's a global variable +alert(i); // 10, "i" is visible after loop, it's a global variable +alert(one); // 1, "one" is visible after loop, it's a global variable */!* ``` @@ -83,7 +85,7 @@ sayHi(); alert(phrase); // Error: phrase is not defined (Check the Developer Console) ``` -As we can see, `var` pierces through `if`, `for` or other code blocks. That's because a long time ago in JavaScript blocks had no Lexical Environments. And `var` is a remnant of that. +As we can see, `var` pierces through `if`, `for` or other code blocks. That's because a long time ago in JavaScript, blocks had no Lexical Environments, and `var` is a remnant of that. ## "var" tolerates redeclarations @@ -203,11 +205,11 @@ sayHi(); Because all `var` declarations are processed at the function start, we can reference them at any place. But variables are undefined until the assignments. -In both examples above `alert` runs without an error, because the variable `phrase` exists. But its value is not yet assigned, so it shows `undefined`. +In both examples above, `alert` runs without an error, because the variable `phrase` exists. But its value is not yet assigned, so it shows `undefined`. -### IIFE +## IIFE -As in the past there was only `var`, and it has no block-level visibility, programmers invented a way to emulate it. What they did was called "immediately-invoked function expressions" (abbreviated as IIFE). +In the past, as there was only `var`, and it has no block-level visibility, programmers invented a way to emulate it. What they did was called "immediately-invoked function expressions" (abbreviated as IIFE). That's not something we should use nowadays, but you can find them in old scripts. @@ -216,22 +218,22 @@ An IIFE looks like this: ```js run (function() { - let message = "Hello"; + var message = "Hello"; alert(message); // Hello })(); ``` -Here a Function Expression is created and immediately called. So the code executes right away and has its own private variables. +Here, a Function Expression is created and immediately called. So the code executes right away and has its own private variables. -The Function Expression is wrapped with parenthesis `(function {...})`, because when JavaScript meets `"function"` in the main code flow, it understands it as the start of a Function Declaration. But a Function Declaration must have a name, so this kind of code will give an error: +The Function Expression is wrapped with parenthesis `(function {...})`, because when JavaScript engine encounters `"function"` in the main code, it understands it as the start of a Function Declaration. But a Function Declaration must have a name, so this kind of code will give an error: ```js run -// Try to declare and immediately call a function +// Tries to declare and immediately call a function function() { // <-- Error: Function statements require a function name - let message = "Hello"; + var message = "Hello"; alert(message); // Hello @@ -277,7 +279,7 @@ In all the above cases we declare a Function Expression and run it immediately. There are two main differences of `var` compared to `let/const`: -1. `var` variables have no block scope, they are visible minimum at the function level. +1. `var` variables have no block scope; their visibility is scoped to current function, or global, if declared outside function. 2. `var` declarations are processed at function start (script start for globals). There's one more very minor difference related to the global object, that we'll cover in the next chapter.