| Feature | var | let | const |
|---|---|---|---|
| Scope | Function-scoped | Block-scoped | Block-scoped |
| Re-declaration | Allowed within the same scope | Not allowed in the same scope | Not allowed in the same scope |
| Re-assignment | Allowed | Allowed | Not allowed after initialization |
| Initialization | Can be declared without initialization | Can be declared without initialization | Must be initialized at the time of declaration |
| Hoisting | Hoisted but initialized to undefined | Hoisted but not initialized | Hoisted but not initialized |
| Type | Function Scope | Block Scope |
|---|---|---|
| var | Variables are scoped to the enclosing function. | Does not support block scope. A var inside a block (if, for, etc.) leaks into the enclosing function or global scope. |
| let / const | Not function-scoped. | Variables are confined to the block they are declared in. |
Re-déclaration
if (true) {
var x = 10;
let y = 20;
const z = 30;
}
console.log(x); // 10 (accessible because of function scope)
console.log(y); // ReferenceError (block-scoped)
console.log(z); // ReferenceError (block-scoped)
| Feature | var | let | const |
|---|---|---|---|
| Re-declaration | Allowed | Not allowed | Not allowed |
| Re-assignment | Allowed | Allowed | Not allowed |
Exemple :
if (true) {
var x = 10;
let y = 20;
const z = 30;
}
console.log(x); // 10 (accessible because of function scope)
console.log(y); // ReferenceError (block-scoped)
console.log(z); // ReferenceError (block-scoped)
| Type | Hoisting Behavior |
|---|---|
| var | Hoisted to the top of the scope but initialized as undefined. |
| let | Hoisted but not initialized. Accessing it before declaration causes a ReferenceError. |
| const | Hoisted but not initialized. Must be initialized at the time of declaration. |
Comportement de levage
// Re-declaration var a = 10; var a = 20; // Allowed let b = 30; // let b = 40; // SyntaxError: Identifier 'b' has already been declared const c = 50; // const c = 60; // SyntaxError: Identifier 'c' has already been declared // Re-assignment a = 15; // Allowed b = 35; // Allowed // c = 55; // TypeError: Assignment to constant variable
| Feature | let and const |
|---|---|
| Block Scope | Both are confined to the block in which they are declared. |
| No Hoisting Initialization | Both are hoisted but cannot be accessed before initialization. |
| Better Practice | Preferred over var for predictable scoping. |
| Scenario | Recommended Keyword |
|---|---|
| Re-declare variables or use function scope | var (generally avoid unless necessary for legacy code). |
| Variables that may change | let (e.g., counters, flags, intermediate calculations). |
| Variables that should not change | const (e.g., configuration settings, fixed values). |
Le levage est le comportement par défaut de JavaScript consistant à déplacer les déclarations vers le haut de leur portée pendant la phase de compilation.
| Type | Hoisting | Access Before Declaration |
|---|---|---|
| var | Hoisted and initialized to undefined. | Allowed but value is undefined. |
| let | Hoisted but not initialized. | Causes a ReferenceError. |
| const | Hoisted but not initialized. | Causes a ReferenceError. |
Exemple :
if (true) {
var x = 10;
let y = 20;
const z = 30;
}
console.log(x); // 10 (accessible because of function scope)
console.log(y); // ReferenceError (block-scoped)
console.log(z); // ReferenceError (block-scoped)
JavaScript a différents types de données classés en types Primitifs et Non primitifs (référence). Voici une explication de chacun avec des exemples et des différences :
Les types primitifs sont immuables, ce qui signifie que leurs valeurs ne peuvent pas être modifiées après leur création. Ils sont stockés directement en mémoire.
| Data Type | Example | Description |
|---|---|---|
| String | "hello", 'world' | Represents a sequence of characters (text). Enclosed in single (''), double (""), or backticks (). |
| Number | 42, 3.14, NaN | Represents both integers and floating-point numbers. Includes NaN (Not-a-Number) and Infinity. |
| BigInt | 123n, 9007199254740991n | Used for numbers larger than Number.MAX_SAFE_INTEGER (2^53 - 1). Add n to create a BigInt. |
| Boolean | true, false | Represents logical values, used in conditions to represent "yes/no" or "on/off". |
| Undefined | undefined | Indicates a variable has been declared but not assigned a value. |
| Null | null | Represents an intentional absence of value. Often used to reset or clear a variable. |
| Symbol | Symbol('id') | Represents a unique identifier, mainly used as property keys for objects to avoid collisions. |
Les types non primitifs sont mutables et stockés par référence. Ils sont utilisés pour stocker des collections de données ou des entités plus complexes.
| Data Type | Example | Description |
|---|---|---|
| Object | {name: 'John', age: 30} | A collection of key-value pairs. Keys are strings (or Symbols), and values can be any type. |
| Array | [1, 2, 3, "apple"] | A list-like ordered collection of values. Access elements via index (e.g., array[0]). |
| Function | function greet() {} | A reusable block of code that can be executed. Functions are first-class citizens in JavaScript. |
| Date | new Date() | Represents date and time. Provides methods for manipulating dates and times. |
| RegExp | /pattern/ | Represents regular expressions used for pattern matching and string searching. |
| Map | new Map() | A collection of key-value pairs where keys can be of any type, unlike plain objects. |
| Set | new Set([1, 2, 3]) | A collection of unique values, preventing duplicates. |
| WeakMap | new WeakMap() | Similar to Map, but keys are weakly held, meaning they can be garbage-collected. |
| WeakSet | new WeakSet() | Similar to Set, but holds objects weakly to prevent memory leaks. |
| Aspect | Primitive Types | Non-Primitive Types |
|---|---|---|
| Mutability | Immutable: Values cannot be changed. | Mutable: Values can be modified. |
| Storage | Stored directly in memory. | Stored as a reference to a memory location. |
| Copy Behavior | Copied by value (creates a new value). | Copied by reference (points to the same object). |
| Examples | string, number, boolean, etc. | object, array, function, etc. |
JavaScript permet aux variables de contenir des valeurs de différents types au moment de l'exécution :
if (true) {
var x = 10;
let y = 20;
const z = 30;
}
console.log(x); // 10 (accessible because of function scope)
console.log(y); // ReferenceError (block-scoped)
console.log(z); // ReferenceError (block-scoped)
// Re-declaration var a = 10; var a = 20; // Allowed let b = 30; // let b = 40; // SyntaxError: Identifier 'b' has already been declared const c = 50; // const c = 60; // SyntaxError: Identifier 'c' has already been declared // Re-assignment a = 15; // Allowed b = 35; // Allowed // c = 55; // TypeError: Assignment to constant variable
console.log(a); // undefined (hoisted) var a = 10; console.log(b); // ReferenceError (temporal dead zone) let b = 20; console.log(c); // ReferenceError (temporal dead zone) const c = 30;
| Expression | Result |
|---|---|
| typeof "hello" | "string" |
| typeof 42 | "number" |
| typeof 123n | "bigint" |
| typeof true | "boolean" |
| typeof undefined | "undefined" |
| typeof null | "object" |
| typeof Symbol() | "symbol" |
| typeof {} | "object" |
| typeof [] | "object" |
| typeof function(){} | "function" |
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