console.log("Hello, World!");
In JavaScript, variables are containers that store data values. This guide will walk you through how to declare, initialize, and work with variables in JavaScript, along with key rules and concepts.
Declaration:
Variables in JavaScript can be declared using var, let, or const.
var variableName; let anotherVariable;
Initialization:
A variable can be initialized with a value at the time of declaration or later.
variableName = value;
var number = 10; let name = "John";
Declaration and Initialization Combined:
var age = 25; let salary = 50000.0;
Using const for Constants:
const is used to declare variables that cannot be reassigned once initialized.
const pi = 3.14159;
var greeting = "Hello";
let count = 5;
const url = "https://example.com";
Examples:
let age; const $price = 9.99; var _isLoggedIn = true; let year2024;
var globalVar = "I am global";
function myFunction() { var functionScopedVar = "I exist only within this function"; }
if (true) { let blockScopedVar = "I exist only within this block"; }
In JavaScript, variable declarations (but not initializations) are hoisted to the top of their scope. This means a variable can be used before it is declared.
console.log(myVar); // Outputs: undefined var myVar = 10;
However, let and const do not allow access before declaration (they are in a "temporal dead zone").
console.log(myLet); // ReferenceError let myLet = 10;
Constants in JavaScript are declared using const. They cannot be reassigned after their initial value is set.
Example:
const DAYS_IN_WEEK = 7; const PI = 3.14159;
However, if the value is an object or an array, its properties or elements can still be modified.
const person = { name: "John" }; person.name = "Doe"; // This is allowed
Variables declared with var, let, or const but not initialized will have the value undefined.
Example:
let num; // default value is undefined console.log(num); // Outputs: undefined
You can declare multiple variables in a single statement, separated by commas. You can also initialize them at the time of declaration.
Examples:
// Declaring multiple variables without values let a, b, c; // Declaring and initializing variables let x = 10, y = 20, z = 30;
You can assign the same value to multiple variables by chaining the assignment operator.
Example:
let m, n, o; m = n = o = 50;
Comments are non-executable statements that help describe and explain the code. They are useful for making the code more readable and maintainable. JavaScript supports two types of comments:
Single-line comments start with two forward slashes (//). Everything following // on that line is considered a comment and will be ignored by the JavaScript engine.
Syntax:
// This is a single-line comment let x = 10; // x is initialized to 10
Multi-line comments start with /* and end with */. Everything between /* and */ is considered a comment, and it can span multiple lines.
Syntax:
/* This is a multi-line comment. It can span multiple lines. */ let y = 20; /* y is initialized to 20 */
Displaying results and interacting with users is fundamental in programming. Here's a comprehensive guide on performing basic output operations in a JavaScript environment (e.g., Node.js):
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);
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);
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:
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:
// Represents both integer and floating-point numbers let num = 42; let pi = 3.14159;
// Represents integers with arbitrary precision let largeNumber = 123456789012345678901234567890n;
// Represents a sequence of characters let greeting = "Hello, World!"; let singleChar = 'A';
// Represents true or false values let isValid = true; let isComplete = false;
// Represents a variable that has been declared but not assigned a value let value; console.log(value); // Outputs: undefined
// Represents the intentional absence of any object value let result = null;
// Represents a unique and immutable value used as an object property key let uniqueSymbol = Symbol('description');
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.
Syntax:
let person = { name: "Alice", age: 30 };
Syntax:
let numbers = [1, 2, 3, 4, 5];
Syntax:
function greet(name) { return "Hello, " + name; }
Syntax:
let today = new Date();
Syntax:
let pattern = /ab+c/;
JavaScript performs type conversion automatically (implicit conversion) but also allows explicit conversion.
let result = "The number is " + 42; // "The number is 42"
let str = String(123); // "123" let num = Number("456"); // 456
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:
You can create a String using string literals or by creating instances of the String object.
let str1 = "Hello"; // Using double quotes let str2 = 'World'; // Using single quotes let str3 = `Hello, ${str2}`; // Using template literals
let str4 = new String(); // Empty string let str5 = new String("JavaScript"); // Using constructor with initial value
String objects are immutable, which means once created, their values cannot be changed.
Example:
let immutableStr = "Hello"; immutableStr = immutableStr + " World"; // Creates a new String object console.log(immutableStr); // Output: Hello World
You can concatenate strings using the + operator or template literals. You can also concatenate numbers with strings.
Example:
let str1 = "Hello"; let str2 = "World"; let concatStr = str1 + " " + str2; // Using + let concatStr2 = `${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
You can get the length of a String using the length property.
Example:
let str = "JavaScript"; let length = str.length; // length is 10
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
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)
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
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 are symbols used to perform operations on values and variables. They are categorized based on their functionality.
Arithmetic operators are used for basic mathematical operations.
Operator | Name | Description | Example |
---|---|---|---|
+ | Addition | Adds two operands | x + y |
- | Subtraction | Subtracts the right operand from the left | x - y |
* | Multiplication | Multiplies two operands | x * y |
/ | Division | Divides the left operand by the right operand | x / y |
% | Modulus | Returns the remainder of the division | x % y |
++ | Increment | Increases the value of operand by 1 | x++ or ++x |
-- | Decrement | Decreases the value of operand by 1 | x-- or --x |
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 are used to assign values to variables and perform operations.
Operator | Name | Description | Example |
---|---|---|---|
= | Assignment | Assigns the value on the right to the variable on the left | x = 5 |
+= | Addition | Adds right operand to the left operand and assigns the result to the left | x += 3 |
-= | Subtraction | Subtracts right operand from the left operand and assigns the result to the left | x -= 3 |
*= | Multiplication | Multiplies right operand with the left operand and assigns the result to the left | x *= 3 |
/= | Division | Divides left operand by right operand and assigns the result to the left | x /= 3 |
%= | Modulus | Computes modulus of left operand with right operand and assigns the result to the left | x %= 3 |
Example:
let x = 10; x += 5; console.log(x); // Output: 15
Comparison operators evaluate conditions and return Boolean values.
Operator | Name | Description | Example |
---|---|---|---|
== | Equal | Checks if two operands are equal | x == y |
=== | Strict Equal | Checks if two operands are equal and of the same type | x === y |
!= | Not Equal | Checks if two operands are not equal | x != y |
!== | Strict Not Equal | Checks if two operands are not equal or not of the same type | x !== y |
> | Greater Than | Checks if left operand is greater than right | x > y |
< | Less Than | Checks if left operand is less than right | x < y |
>= | Greater Than or Equal | Checks if left operand is greater than or equal to right | x >= y |
<= | Less Than or Equal | Checks if left operand is less than or equal to right | x <= y |
Example:
let a = 5; let b = 10; console.log(a == b); // Output: false console.log(a < b); // Output: true
Logical operators combine Boolean expressions and return Boolean values.
Operator | Description | Example |
---|---|---|
&& | Logical AND | x < 5 && x < 10 |
|| | Logical OR | x < 5 || x < 4 |
! | Logical NOT | !(x < 5 && x < 10) |
Example:
let x = 3; console.log(x < 5 && x < 10); // Output: true console.log(x < 5 || x < 2); // Output: true
Bitwise operators are used to perform bitwise operations on integers.
Operator | Name | Description | Example |
---|---|---|---|
& | AND | Sets each bit to 1 if both bits are 1 | x & y |
| | OR | Sets each bit to 1 if one of two bits is 1 | x | y |
^ | XOR | Sets each bit to 1 if only one of two bits is 1 | x ^ y |
~ | NOT | Inverts all the bits | ~x |
<< | Left Shift | Shifts bits to the left | x << 2 |
>> | Right Shift | Shifts bits to the right | x >> 2 |
>>> | Unsigned Right Shift | Shifts bits to the right and fills with zeros | x >>> 2 |
Example:
let x = 5; let y = 3; console.log(x & y); // Output: 1 console.log(x | y); // Output: 7
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 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.
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.
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); }
let x = 10; if (x > 5) { console.log("x is greater than 5"); // Output: x is greater than 5 }
let x = 3; if (x % 2 === 0) { console.log("x is even"); } else { console.log("x is odd"); // Output: x is odd }
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 }
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 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.
A for loop is used when the number of iterations is known beforehand. It includes initialization, condition, and update expressions.
for (initialization; condition; update) { // Code to be executed }
for (let i = 0; i < 5; i++) { console.log("Iteration: " + i); }
The for...in loop iterates over the enumerable properties of an object. It is commonly used to loop through the keys of an object.
for (key in object) { // Code to be executed }
let person = { name: "Alice", age: 25, city: "New York" }; for (let key in person) { console.log(key + ": " + person[key]); }
The for...of loop iterates over iterable objects like arrays, strings, maps, and sets. It provides a convenient way to loop through elements.
for (element of iterable) { // Code to be executed }
let numbers = [1, 2, 3, 4, 5]; for (let number of numbers) { console.log(number); }
A while loop is used when the number of iterations is not known beforehand, and the loop continues until a specified condition is false.
while (condition) { // Code to be executed }
let i = 0; while (i < 5) { console.log("Iteration: " + i); i++; }
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.
do { // Code to be executed } while (condition);
let i = 0; do { console.log("Iteration: " + i); i++; } while (i < 5);
Loops can be nested within other loops, which is useful for iterating over multi-dimensional arrays or performing more complex iterations.
for (let i = 0; i < 3; i++) { for (let j = 0; j < 3; j++) { console.log("i: " + i + ", j: " + j); } }
break and continue statements are used to alter the flow of loops:
for (let i = 0; i < 5; i++) { if (i === 3) { break; } console.log("Iteration: " + i); }
for (let i = 0; i < 5; i++) { if (i === 3) { continue; } console.log("Iteration: " + i); }
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.
A function in JavaScript is defined using the function keyword, followed by a name, parameters (optional), and a block of code.
function functionName(parameters) { // Code to be executed }
function greet() { return "Hello, World!"; } console.log(greet()); // Output: Hello, World!
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.
function greet() { return "Hello, World!"; } console.log(greet()); // Output: Hello, World!
Functions can take parameters, which are used to pass values into the function.
function add(a, b) { return a + b; } console.log(add(5, 3)); // Output: 8
A function can return a value using the return statement. The return type of the function must match the type of the value returned.
function multiply(a, b) { return a * b; } console.log(multiply(4, 7)); // Output: 28
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.
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
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.
function factorial(n) { if (n === 0) { return 1; } else { return n * factorial(n - 1); } } console.log(factorial(5)); // Output: 120
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.
const greet = function() { return "Hello, World!"; }; console.log(greet()); // Output: Hello, World!
Arrow functions provide a concise syntax for writing functions. They do not have their own this context and are often used for shorter functions.
const functionName = (parameters) => { // Code to be executed };
const add = (a, b) => a + b; console.log(add(5, 3)); // Output: 8
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.
(function() { // Code to be executed })();
(function() { const message = "Hello, World!"; console.log(message); })(); // Output: Hello, World!
A function expression involves assigning a function to a variable. Function expressions can be named or anonymous.
const square = function(x) { return x * x; }; console.log(square(4)); // Output: 16
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.
function functionName(parameter = defaultValue) { // Code to be executed }
function greet(name = "Guest") { return "Hello, " + name + "!"; } console.log(greet()); // Output: Hello, Guest! console.log(greet("Alice")); // Output: Hello, Alice!
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.
function functionName(...rest) { // Code to be executed }
function sum(...numbers) { return numbers.reduce((acc, curr) => acc + curr, 0); } console.log(sum(1, 2, 3, 4, 5)); // Output: 15
In JavaScript, functions have an arguments object that contains all the arguments passed to the function. This object is not available in arrow functions.
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.
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 }
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
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)
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)
The above is the detailed content of JavaScript Basics. For more information, please follow other related articles on the PHP Chinese website!