In JavaScript, the terms "ES5" and "ES6" refer to two different versions of the ECMAScript standard, which is the formal definition of the core of the JavaScript programming language. ES5 (ECMAScript 5) was standardized in 2009, while ES6 (ECMAScript 2015), also known as ES2015, was standardized in 2015. Both versions have different features and behaviors, including how they handle variable declarations and hoisting.
### Function Scope in ES5
In ES5, JavaScript has function scope, which means that variables declared within a function are only accessible within that function. This is in contrast to block scope, where variables would be accessible within any block (e.g., loops, conditionals) they are declared in.
### Hoisting in ES5
Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope before code execution. In ES5, both `var` declarations and function declarations are hoisted:
- **Variable Hoisting**: When you declare a variable with `var`, the declaration is hoisted to the top of the function scope, but the initialization is not. This can lead to cases where you might get `undefined` if you try to access the variable before it's explicitly declared and initialized in the code.
- **Function Hoisting**: Function declarations are fully hoisted, meaning both the declaration and the definition are moved to the top of their containing scope.
### ES6 and Hoisting
ES6 introduced `let` and `const` for variable declarations, which have block scope instead of function scope. Unlike `var`, `let` and `const` declarations are not hoisted in the same way:
- **`let` and `const` Hoisting**: These declarations are not hoisted to the top of their block scope. Instead, they are in a "temporal dead zone" (TDZ) from the start of the block until the point where they are declared. Accessing them before the declaration results in a `ReferenceError`.
- **Function Hoisting in ES6**: Function declarations are still hoisted in ES6, but the behavior of function expressions, especially those assigned to variables, has changed. For example, a function expression assigned to a variable using `let` or `const` will not be hoisted.
### Example to Clarify
Here is an example to illustrate the differences:
```javascript
// ES5 with var
function foo() {
console.log(bar); // undefined, not a ReferenceError, because of hoisting
var bar = 1;
}
// ES6 with let
function fooES6() {
console.log(bar); // ReferenceError: bar is not defined, because 'let' is in TDZ
let bar = 1;
}
```
In the ES5 example, `bar` is hoisted to the top of the function, and since it's declared with `var`, it is accessible but initialized with `undefined`. In the ES6 example, `bar` is not accessible before the line where it is declared, due to the temporal dead zone.
It's important to note that while ES6 introduced `let` and `const`, `var` is still part of the language and follows the same hoisting rules as in ES5. Developers often prefer `let` and `const` for their block scoping and immutability features, which can lead to more predictable and maintainable code.
