Bases de JavaScript

Javascript

console.log("Hello, World!");

Création de variables

En JavaScript, les variables sont des conteneurs qui stockent les valeurs des données. Ce guide vous expliquera comment déclarer, initialiser et utiliser des variables en JavaScript, ainsi que les règles et concepts clés.

Syntaxe pour la déclaration et l'initialisation de variables

Déclaration :
Les variables en JavaScript peuvent être déclarées en utilisant var, let ou const.

var variableName;
let anotherVariable;

Initialisation :
Une variable peut être initialisée avec une valeur au moment de la déclaration ou ultérieurement.

variableName = value;

var number = 10;
let name = "John";

Déclaration et initialisation combinées :

var age = 25;
let salary = 50000.0;

Utilisation de const pour les constantes :
const est utilisé pour déclarer des variables qui ne peuvent pas être réaffectées une fois initialisées.

const pi = 3.14159;

Déclarations de variables : var, let et const

• var: Déclare une variable de portée fonction ou de portée globale si elle est déclarée en dehors d'une fonction. Il peut être re-déclaré et mis à jour.

  var greeting = "Hello";

• let : Déclare une variable de portée bloc, ce qui signifie qu'elle n'est accessible que dans le bloc dans lequel elle est déclarée. Il peut être mis à jour mais pas redéclaré dans le même périmètre.

  let count = 5;

• const : Déclare une variable en lecture seule de portée bloc. Il doit être initialisé au moment de la déclaration et ne peut être réaffecté.

  const url = "https://example.com";

Règles pour les noms de variables

1. Les noms de variables doivent commencer par une lettre, un trait de soulignement (_) ou un signe dollar ($).
2. Les caractères suivants peuvent être des lettres, des chiffres, des traits de soulignement ou des signes dollar.
3. Les noms de variables sont sensibles à la casse (myVar et myvar sont différents).
4. Les noms de variables ne peuvent pas être des mots-clés réservés en JavaScript (par exemple, let, function, class).

Exemples :

let age;
const $price = 9.99;
var _isLoggedIn = true;
let year2024;

Portée des variables

• Portée globale : Une variable déclarée en dehors de toute fonction ou bloc a une portée globale et est accessible depuis n'importe où dans le code.

  var globalVar = "I am global";

• Portée de la fonction : Les variables déclarées avec var à l'intérieur d'une fonction ont une portée de fonction.

  function myFunction() {
      var functionScopedVar = "I exist only within this function";
  }

• Portée du bloc : Les variables déclarées avec let ou const à l'intérieur d'un bloc {} ont une portée de bloc.

  if (true) {
      let blockScopedVar = "I exist only within this block";
  }

Levage

En JavaScript, les déclarations de variables (mais pas les initialisations) sont hissées au sommet de leur portée. Cela signifie qu'une variable peut être utilisée avant d'être déclarée.

console.log(myVar); // Outputs: undefined
var myVar = 10;

Cependant, let et const n'autorisent pas l'accès avant déclaration (ils sont dans une "zone morte temporelle").

console.log(myLet); // ReferenceError
let myLet = 10;

Constantes

Les constantes en JavaScript sont déclarées à l'aide de const. Ils ne peuvent pas être réaffectés une fois leur valeur initiale définie.

Exemple :

const DAYS_IN_WEEK = 7;
const PI = 3.14159;

Cependant, si la valeur est un objet ou un tableau, ses propriétés ou éléments peuvent toujours être modifiés.

const person = { name: "John" };
person.name = "Doe";  // This is allowed

Valeurs par défaut

Les variables déclarées avec var, let ou const mais non initialisées auront la valeur indéfinie.

Exemple :

let num;  // default value is undefined
console.log(num);  // Outputs: undefined

Déclarer plusieurs variables

Vous pouvez déclarer plusieurs variables dans une seule instruction, séparées par des virgules. Vous pouvez également les initialiser au moment de la déclaration.

Exemples :

// Declaring multiple variables without values
let a, b, c;

// Declaring and initializing variables
let x = 10, y = 20, z = 30;

Attribuer une valeur à plusieurs variables

Vous pouvez attribuer la même valeur à plusieurs variables en chaînant l'opérateur d'affectation.

Exemple :

let m, n, o;
m = n = o = 50;

Créer des commentaires

Les commentaires sont des instructions non exécutables qui aident à décrire et à expliquer le code. Ils sont utiles pour rendre le code plus lisible et maintenable. JavaScript prend en charge deux types de commentaires :

1. Commentaires sur une seule ligne

Les commentaires sur une seule ligne commencent par deux barres obliques (//). Tout ce qui suit // sur cette ligne est considéré comme un commentaire et sera ignoré par le moteur JavaScript.

Syntaxe :

// This is a single-line comment
let x = 10; // x is initialized to 10

2. Commentaires sur plusieurs lignes

Les commentaires sur plusieurs lignes commencent par /* et se terminent par */. Tout ce qui se trouve entre /* et */ est considéré comme un commentaire et peut s'étendre sur plusieurs lignes.

Syntaxe :

/*
 This is a multi-line comment.
 It can span multiple lines.
*/
let y = 20; /* y is initialized to 20 */

Sortie de base

Afficher les résultats et interagir avec les utilisateurs est fondamental en programmation. Voici un guide complet sur la réalisation d'opérations de sortie de base dans un environnement JavaScript (par exemple, Node.js) :

1. Output (Printing to Console)

To display output on the console, use the console.log() method. This method prints the string representation of the given data to the standard output (usually the console) followed by a newline.

Syntax:

console.log("Hello, World!");

Example:

let number = 10;
console.log("The number is: " + number);

2. Formatting Output

JavaScript allows you to format output using template literals or string concatenation.

Using Template Literals:
Template literals enable embedded expressions and multi-line strings. They are enclosed in backticks (`).

Syntax:

let name = "Alice";
let age = 30;
console.log(`Name: ${name}, Age: ${age}`);

Using String Concatenation:
You can also format output using the + operator.

Syntax:

let name = "Alice";
let age = 30;
console.log("Name: " + name + ", Age: " + age);

Data Types

JavaScript is a dynamically-typed language, meaning variables can hold values of any type and can change types during runtime. JavaScript provides several categories of data types:

1. Primitive Data Types

Primitive data types are the most basic types of data in JavaScript. They are immutable and are not objects. There are seven primitive data types:

Number Type:

// Represents both integer and floating-point numbers
let num = 42;
let pi = 3.14159;

BigInt Type:

// Represents integers with arbitrary precision
let largeNumber = 123456789012345678901234567890n;

String Type:

// Represents a sequence of characters
let greeting = "Hello, World!";
let singleChar = 'A';

Boolean Type:

// Represents true or false values
let isValid = true;
let isComplete = false;

Undefined Type:

// Represents a variable that has been declared but not assigned a value
let value;
console.log(value); // Outputs: undefined

Null Type:

// Represents the intentional absence of any object value
let result = null;

Symbol Type:

// Represents a unique and immutable value used as an object property key
let uniqueSymbol = Symbol('description');

2. Object Data Types

Object data types are collections of properties, where each property is a key-value pair. Objects can be used to group related data and functionality.

• Object: The most general type of object in JavaScript. It can hold multiple values as properties.

Syntax:

  let person = {
      name: "Alice",
      age: 30
  };

• Array: A special type of object used to store ordered collections of values. Arrays are indexed starting from 0.

Syntax:

  let numbers = [1, 2, 3, 4, 5];

• Function: Functions are also objects in JavaScript. They can be assigned to variables, passed as arguments, and returned from other functions.

Syntax:

  function greet(name) {
      return "Hello, " + name;
  }

• Date: A built-in object for handling dates and times.

Syntax:

  let today = new Date();

• RegExp: A built-in object for working with regular expressions.

Syntax:

  let pattern = /ab+c/;

Type Conversion

JavaScript performs type conversion automatically (implicit conversion) but also allows explicit conversion.

• Implicit Conversion: Automatically converts types during operations.

  let result = "The number is " + 42; // "The number is 42"

• Explicit Conversion: Manually converting types using functions.

  let str = String(123);  // "123"
  let num = Number("456"); // 456

Strings in JavaScript

String is a built-in object that represents a sequence of characters. It is widely used for manipulating text and is immutable, meaning once a String object is created, its value cannot be changed. Here's a comprehensive guide covering everything you need to know about String:

1. Creating Strings

You can create a String using string literals or by creating instances of the String object.

Using String Literals:

let str1 = "Hello"; // Using double quotes
let str2 = 'World'; // Using single quotes
let str3 = `Hello, ${str2}`; // Using template literals

Using String Constructor:

let str4 = new String(); // Empty string
let str5 = new String("JavaScript"); // Using constructor with initial value

2. String Immutability

String objects are immutable, which means once created, their values cannot be changed.

Example:

let immutableStr = "Hello";
immutableStr = immutableStr + " World"; // Creates a new String object
console.log(immutableStr); // Output: Hello World

3. String Concatenation

You can concatenate strings using the + operator or template literals. You can also concatenate numbers with strings.

Example:

let str1 = "Hello";
let str2 = "World";
let concatStr = str1 + " " + str2; // Using +
let concatStr2 = `${str1} ${str2}`; // Using template literals

let num = 42;
let numConcat = `The answer is ${num}`; // Concatenating number with string
console.log(concatStr); // Output: Hello World
console.log(numConcat); // Output: The answer is 42

4. String Length

You can get the length of a String using the length property.

Example:

let str = "JavaScript";
let length = str.length; // length is 10

5. String Comparison

You can compare strings using the === operator for strict equality and locale-aware comparison methods.

Example:

let str1 = "JavaScript";
let str2 = "javascript";
console.log(str1 === str2); // false
console.log(str1.toLowerCase() === str2.toLowerCase()); // true

6. String Interning

While not explicitly handled like in some other languages, strings created using literals may be optimized for performance by the JavaScript engine.

Example:

let str1 = "JavaScript";
let str2 = "JavaScript";
console.log(str1 === str2); // true (interning by engine)

7. Escape Sequences

JavaScript supports escape sequences within strings, such as \n for newline, \t for tab, and \\ for a backslash.

Example:

let escapeStr = "Hello\tWorld\nJavaScript";
console.log(escapeStr);
// Output:
// Hello   World
// JavaScript

8. String Formatting

You can use template literals for formatting.

Example using Template Literals:

let age = 30;
let formattedStr = `I am ${age} years old.`;
console.log(formattedStr);
// Output: I am 30 years old.

let price = 19.95;
let formattedPrice = `The price is ${price.toFixed(2)} dollars.`;
console.log(formattedPrice);
// Output: The price is 19.95 dollars.

Operators in JavaScript

Operators in JavaScript are symbols used to perform operations on values and variables. They are categorized based on their functionality.

Arithmetic Operators

Arithmetic operators are used for basic mathematical operations.

Example:

let a = 10;
let b = 3;
console.log(a + b);   // Output: 13
console.log(a / b);   // Output: 3.3333333333333335
console.log(a % b);   // Output: 1

let x = 5;
x++;
console.log(x);       // Output: 6

let y = 8;
y--;
console.log(y);       // Output: 7

Assignment Operators

Assignment operators are used to assign values to variables and perform operations.

Example:

let x = 10;
x += 5;
console.log(x);   // Output: 15

Comparison Operators

Comparison operators evaluate conditions and return Boolean values.

Example:

let a = 5;
let b = 10;
console.log(a == b);   // Output: false
console.log(a < b);    // Output: true

Logical Operators

Logical operators combine Boolean expressions and return Boolean values.

Example:

let x = 3;
console.log(x < 5 && x < 10);   // Output: true
console.log(x < 5 || x < 2);    // Output: true

Bitwise Operators

Bitwise operators are used to perform bitwise operations on integers.

Example:

let x = 5;
let y = 3;
console.log(x & y);   // Output: 1
console.log(x | y);   // Output: 7

Ternary Operator

The ternary operator ? : provides a shorthand for conditional expressions.

Example:

let age = 20;
let status = (age >= 18) ? "Adult" : "Minor";
console.log(status);   // Output: Adult

Conditions

Conditions are used to execute specific blocks of code based on whether a certain expression evaluates to true or false. They control the flow of a program by allowing different code to run depending on the outcome of conditional expressions. In JavaScript, conditional statements such as if, else if, and else help manage decision-making in the code.

If-Else Statements in JavaScript

In JavaScript, if statements are employed to execute code based on the evaluation of a condition. A condition in JavaScript evaluates to either true or false, determining which block of code will execute. The else and else if statements are used to specify alternative code blocks based on different conditions.

Syntax and Usage

The basic syntax of an if statement in JavaScript is:

if (condition) {
    // Executes if the condition is true
    statement(s);
}

If you need to execute an alternative block of code when the condition is false, use else:

if (condition) {
    // Executes if the condition is true
    statement(s);
} else {
    // Executes if the condition is false
    statement(s);
}

For handling multiple conditions, use else if:

if (condition1) {
    // Executes if condition1 is true
    statement(s);
} else if (condition2) {
    // Executes if condition1 is false and condition2 is true
    statement(s);
} else {
    // Executes if both condition1 and condition2 are false
    statement(s);
}

Examples

1. Simple if statement:

let x = 10;

if (x > 5) {
    console.log("x is greater than 5");  // Output: x is greater than 5
}

1. if-else statement:

let x = 3;

if (x % 2 === 0) {
    console.log("x is even");
} else {
    console.log("x is odd");  // Output: x is odd
}

1. if-else if-else statement:

let x = 20;

if (x > 50) {
    console.log("x is greater than 50");
} else if (x > 30) {
    console.log("x is greater than 30 but less than or equal to 50");
} else {
    console.log("x is 30 or less");  // Output: x is 30 or less
}

Note:

In JavaScript, you can use any expression in a condition, and values that are not strictly true or false can still be evaluated. For example, non-zero numbers, non-empty strings, and objects are considered true, while 0, null, undefined, NaN, and empty strings are considered false. This flexibility allows for concise condition checks without explicitly comparing values to true or false.

Loops

Loops in JavaScript are used to execute a block of code repeatedly based on a specified condition. JavaScript supports several types of loops: for, for...in, for...of, while, and do...while. Each loop serves different purposes and choosing the right loop can help streamline your code and improve its readability.

1. For Loop

A for loop is used when the number of iterations is known beforehand. It includes initialization, condition, and update expressions.

Syntax:

for (initialization; condition; update) {
    // Code to be executed
}

Example:

for (let i = 0; i < 5; i++) {
    console.log("Iteration: " + i);
}

2. For...in Loop

The for...in loop iterates over the enumerable properties of an object. It is commonly used to loop through the keys of an object.

Syntax:

for (key in object) {
    // Code to be executed
}

Example:

let person = { name: "Alice", age: 25, city: "New York" };
for (let key in person) {
    console.log(key + ": " + person[key]);
}

3. For...of Loop

The for...of loop iterates over iterable objects like arrays, strings, maps, and sets. It provides a convenient way to loop through elements.

Syntax:

for (element of iterable) {
    // Code to be executed
}

Example:

let numbers = [1, 2, 3, 4, 5];
for (let number of numbers) {
    console.log(number);
}

4. While Loop

A while loop is used when the number of iterations is not known beforehand, and the loop continues until a specified condition is false.

Syntax:

while (condition) {
    // Code to be executed
}

Example:

let i = 0;
while (i < 5) {
    console.log("Iteration: " + i);
    i++;
}

5. Do...While Loop

A do...while loop is similar to a while loop, but it ensures that the code block is executed at least once because the condition is evaluated after the loop body.

Syntax:

do {
    // Code to be executed
} while (condition);

Example:

let i = 0;
do {
    console.log("Iteration: " + i);
    i++;
} while (i < 5);

6. Nested Loops

Loops can be nested within other loops, which is useful for iterating over multi-dimensional arrays or performing more complex iterations.

Example:

for (let i = 0; i < 3; i++) {
    for (let j = 0; j < 3; j++) {
        console.log("i: " + i + ", j: " + j);
    }
}

7. Break and Continue

break and continue statements are used to alter the flow of loops:

• break exits the loop immediately.
• continue skips the current iteration and proceeds to the next iteration.

Example of break:

for (let i = 0; i < 5; i++) {
    if (i === 3) {
        break;
    }
    console.log("Iteration: " + i);
}

Example of continue:

for (let i = 0; i < 5; i++) {
    if (i === 3) {
        continue;
    }
    console.log("Iteration: " + i);
}

Functions in JavaScript

Functions in JavaScript are blocks of code designed to perform specific tasks. Functions allow you to write reusable code, making your programs more organized and modular. JavaScript functions can be defined using several syntaxes and can handle parameters and return values. Here's a comprehensive guide to understanding and using functions in JavaScript.

1. Function Definition and Syntax

A function in JavaScript is defined using the function keyword, followed by a name, parameters (optional), and a block of code.

Syntax:

function functionName(parameters) {
    // Code to be executed
}

• functionName: The name of the function.
• parameters: A comma-separated list of input parameters. If there are no parameters, leave the parentheses empty.

Example:

function greet() {
    return "Hello, World!";
}

console.log(greet());  // Output: Hello, World!

2. Calling Functions

To execute a function, you call it by using its name followed by parentheses. Functions can be called from within other functions or from different parts of your code.

Example:

function greet() {
    return "Hello, World!";
}

console.log(greet());  // Output: Hello, World!

3. Function Parameters

Functions can take parameters, which are used to pass values into the function.

Example:

function add(a, b) {
    return a + b;
}

console.log(add(5, 3));  // Output: 8

4. Return Values

A function can return a value using the return statement. The return type of the function must match the type of the value returned.

Example:

function multiply(a, b) {
    return a * b;
}

console.log(multiply(4, 7));  // Output: 28

5. Function Overloading

JavaScript does not support method overloading (having multiple functions with the same name but different parameters) directly. Instead, you can use default parameters or check the number and types of arguments inside the function.

Example with argument checking:

function print(value) {
    if (typeof value === "number") {
        console.log("Number: " + value);
    } else if (typeof value === "string") {
        console.log("String: " + value);
    }
}

print(10);          // Output: Number: 10
print("Hello");     // Output: String: Hello

6. Recursive Functions

A function can call itself, a concept known as recursion. Recursion is useful for solving problems that can be broken down into smaller, repetitive tasks.

Example:

function factorial(n) {
    if (n === 0) {
        return 1;
    } else {
        return n * factorial(n - 1);
    }
}

console.log(factorial(5));  // Output: 120

7. Anonymous Functions

An anonymous function is a function that does not have a name. These functions are often used as arguments to other functions or assigned to variables.

Example:

const greet = function() {
    return "Hello, World!";
};

console.log(greet());  // Output: Hello, World!

8. Arrow Functions

Arrow functions provide a concise syntax for writing functions. They do not have their own this context and are often used for shorter functions.

Syntax:

const functionName = (parameters) => {
    // Code to be executed
};

Example:

const add = (a, b) => a + b;

console.log(add(5, 3));  // Output: 8

9. Immediately Invoked Function Expressions (IIFE)

An IIFE is a function that is executed immediately after its definition. It is often used to create a new scope to avoid polluting the global namespace.

Syntax:

(function() {
    // Code to be executed
})();

Example:

(function() {
    const message = "Hello, World!";
    console.log(message);
})();  // Output: Hello, World!

10. Function Expressions

A function expression involves assigning a function to a variable. Function expressions can be named or anonymous.

Example:

const square = function(x) {
    return x * x;
};

console.log(square(4));  // Output: 16

11. Default Parameters

Functions in JavaScript can have default values for parameters. If a parameter is not provided when the function is called, the default value is used.

Syntax:

function functionName(parameter = defaultValue) {
    // Code to be executed
}

Example:

function greet(name = "Guest") {
    return "Hello, " + name + "!";
}

console.log(greet());        // Output: Hello, Guest!
console.log(greet("Alice")); // Output: Hello, Alice!

12. Rest Parameters

Rest parameters allow you to represent an indefinite number of arguments as an array. They must be the last parameter in the function’s parameter list.

Syntax:

function functionName(...rest) {
    // Code to be executed
}

Example:

function sum(...numbers) {
    return numbers.reduce((acc, curr) => acc + curr, 0);
}

console.log(sum(1, 2, 3, 4, 5));  // Output: 15

13. The arguments Object

In JavaScript, functions have an arguments object that contains all the arguments passed to the function. This object is not available in arrow functions.

Example:

function showArguments() {
    console.log(arguments);
}

showArguments(1, "test", true);  // Output: [1, "test", true]

Note: The arguments object is not available in arrow functions; instead, use rest parameters for similar functionality.

Method Parameters in JavaScript

1. Syntax for Defining Method Parameters

In JavaScript, method parameters are defined within parentheses following the method name. Each parameter is declared with its name, and there is no need to specify its type.

function functionName(parameter1, parameter2, ...) {
    // Code to be executed
}

2. Passing Arguments

When invoking a function, you pass arguments to match the parameters defined in the function signature. JavaScript functions can accept any number of arguments, and extra arguments are ignored if they are not specified in the function definition.

function printDetails(name, age) {
    console.log("Name: " + name + ", Age: " + age);
}

printDetails("John", 30); // Passing arguments "John" and 30

3. Pass by Value

JavaScript passes primitive data types (e.g., numbers, strings) by value. This means a copy of the actual value is passed to the function. Changes made to the parameter inside the function do not affect the original value.

function modifyValue(num) {
    num = num + 10; // Changes made to num inside the function
}

let x = 5;
modifyValue(x);
console.log(x); // Output: 5 (unchanged)

4. Pass by Reference

JavaScript objects and arrays are passed by reference. This means that the reference to the object or array is passed, and changes to the object's properties or array elements will affect the original object or array.

function modifyObject(obj) {
    obj.value = obj.value + 10; // Modifying obj inside the function
}

let myObject = { value: 5 };
modifyObject(myObject);
console.log(myObject.value); // Output: 15 (modified)

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