Learn JS step by step

Variables

Declaration, assignment and scope of variables in JavaScript:

Variable declaration

• Use var, let or const to declare a variable.

Assigning variables

Use = to assign a value to a variable.

Variable scope

• Global variables: Declared outside all functions, accessible everywhere.
• Local variables: Declared inside a function, accessible only inside it.
• Block-scoped variables: let and const are limited to the {} block where they are declared. A closure is different: it is a function that retains access to its lexical environment.
• Hoisting: var, let and const bindings are created before execution reaches their declaration. var initially contains undefined; let and const remain inaccessible in the temporal dead zone until initialization.

Recommendations:

• Preference for let and const: Use let for reassignable variables, const for variables whose value does not change.
• Limit scope: Declare variables in the narrowest possible scope.
• Name variables explicitly: Use meaningful names to facilitate understanding.
• Initialize the variables: Assign an initial value upon declaration.

Data types

JavaScript has seven primitive types (string, number, bigint, boolean, undefined, symbol, null). Arrays, functions, dates and regular expressions are objects or callable objects.

FlashCard

#	Data type	Description	Examples
1.	Number	Represents numeric values ​​	42, 3.14, -10
2.	String	Represents a sequence of characters	'Hello', "JavaScript"
3.	Boolean	Represents a logical value (true or false)	true, false
4.	Table	Ordered collection of elements	[1, 2, 3], ['a', 'b', 'c']
5.	Object	Entity with properties and methods	name: 'John', age: 25
6.	Undefined	Indicates that a variable has no assigned value	undefined
7.	Null	Indicates intentional absence of value	null
8.	Symbol	Unique identifiers introduced in ES6	Symbol('description')
9.	Date Object	Represents a specific date and time	new Date()
10.	RegExp object	Used for regular expressions	/pattern/, new RegExp('pattern')
11.	Function	Reusable code block called with name and parameters	function addition(a, b) return a + b;

Operators

#	Operator	Description
1.	Arithmetic operators	+, -, *, /, %
2.	Comparison operators

| 3. | Logical operators | &&, ||, ! | | 4. | Assignment operators | =, +=, -=, *=, /=, %= | | 5. | Increment operators | ++, -- | | 6. | Concatenation operator | + | | 7. | Ternary operator | condition ? expression1: expression2 | | 8. | Bitwise operators | | | 9. | String concatenation operator | += | | 10. | Membership operators | in, instanceof | | 11. | Selection operator | . | | 12. | Type Test Operators | typeof, instanceof | | 13. | Shift operators | | | 14. | Delete operator | delete | | 15. | Grouping and access | (), [], ., ?. |

Control structure

#	Control structure	Description
1.	Conditional structure if/else	Allows you to execute a block of code based on a condition.
2.	Switch conditional structure	Allows you to select a block of code to execute among several cases.
3.	for loop	Executes a block of code a certain number of times, based on a check condition.
4.	while loop	Executes a block of code as long as a given condition is true.
5.	do...while loop	Executes a block of code at least once, then continues until a given condition is true.
6.	Loop for...in	Iterates through the enumerable properties of an object.
7.	for...of loop	Iterates through the elements of an iterable structure (array, string, etc.).
8.	Ternary conditional structure	Allows you to perform a condition in a single line.
9.	Break instructions	Ends execution of a loop or conditional statement.
10.	Continued instruction	Moves to the next iteration of a loop.

Functions

#	Function	Description	Courses and examples

| 1. | Declarative function | Declares a function with the keyword function. | Course: function nomFonction() { // Code }
Example: function afficherMessage() { console.log("Bonjour !"); } |

| 2. | Function with parameters | Allows you to pass values ​​to a function using parameters. | Course: function nomFonction(param1, param2) { // Code } Example: function additionner(a, b) { return a + b; } | | 3. | Function with return value | Returns a value calculated by the function with the return keyword. | Course: function nomFonction() { return valeur; } Example: function carre(nombre) { return nombre * nombre; } | | 4. | Anonymous function | Unnamed function, often used as an argument to another function. | Course: (function() { // Code })(); Example: (function() { console.log("Fonction anonyme exécutée !"); })(); | | 5. | Arrow function | Simplified syntax for declaring a function. | Course: () => { // Code } Example: const multiplier = (a, b) => a * b; | | 6. | Recursive function | Calls itself to perform repetitive operations. | Course: function nomFonction() { nomFonction(); } Example: function compteur(numero) { if (numero > 0) { console.log(numero); compteur(numero - 1); } } | | 7. | Callback function | Function passed as an argument to another function to be called later. | Course: function fonctionPrincipale(callback) { // Code ; callback(); } Example: function afficherMessage(callback) { console.log("Bonjour !"); callback(); } afficherMessage(() => console.log("Fin du programme")); | | 8. | Self-executing function (IIFE) | Runs immediately after setting. | Course: (function() { // Code })(); Example: (function() { console.log("Fonction auto-exécutante !"); })(); |

Declarative function

• A declarative function is defined using the keyword function.
• It can have a name to be reused later.

function afficherMessage() {
  console.log("Bonjour !");
}

afficherMessage(); // Appel de la fonction