JavaScript 基础知识

JavaScript

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

创建变量

在 JavaScript 中，变量是存储数据值的容器。本指南将引导您了解如何在 JavaScript 中声明、初始化和使用变量，以及关键规则和概念。

变量声明和初始化的语法

声明：
JavaScript 中的变量可以使用 var、let 或 const 来声明。

var variableName;
let anotherVariable;

初始化：
变量可以在声明时或之后用一个值进行初始化。

variableName = value;

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

声明和初始化组合：

var age = 25;
let salary = 50000.0;

使用 const 表示常量：
const 用于声明一旦初始化就无法重新赋值的变量。

const pi = 3.14159;

变量声明：var、let 和 const

• var: 声明一个函数范围的变量，如果在函数外部声明，则声明一个全局范围的变量。可以重新声明和更新。

  var greeting = "Hello";

• let: 声明一个块作用域变量，这意味着它只能在声明的块内访问。它可以更新，但不能在同一范围内重新声明。

  let count = 5;

• const: 声明一个块作用域的只读变量。它必须在声明时初始化并且不能重新分配。

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

变量名称规则

1. 变量名称必须以字母、下划线 (_) 或美元符号 ($) 开头。
2. 后续字符可以是字母、数字、下划线或美元符号。
3. 变量名称区分大小写（myVar 和 myvar 不同）。
4. 变量名不能是 JavaScript 中的保留关键字（例如 let、function、class）。

示例：

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

变量的范围

• 全局作用域： 在任何函数或块之外声明的变量具有全局作用域，可以从代码中的任何位置访问。

  var globalVar = "I am global";

• 函数作用域： 在函数内用 var 声明的变量是函数作用域的。

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

• 块作用域： 在块 {} 内用 let 或 const 声明的变量是块作用域的。

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

吊装

在 JavaScript 中，变量声明（但不是初始化）被提升到其作用域的顶部。这意味着变量可以在声明之前使用。

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

但是，let 和 const 不允许在声明之前进行访问（它们处于“临时死区”）。

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

常数

JavaScript 中的常量是使用 const 声明的。设置初始值后就无法重新分配它们。

示例：

const DAYS_IN_WEEK = 7;
const PI = 3.14159;

但是，如果值是对象或数组，则仍然可以修改其属性或元素。

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

默认值

使用 var、let 或 const 声明但未初始化的变量将具有未定义的值。

示例：

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

声明多个变量

您可以在一条语句中声明多个变量，用逗号分隔。您也可以在声明时初始化它们。

示例：

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

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

将一个值分配给多个变量

您可以通过链接赋值运算符将相同的值分配给多个变量。

示例：

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

创建评论

注释是不可执行的语句，有助于描述和解释代码。它们对于使代码更具可读性和可维护性很有用。 JavaScript 支持两种类型的注释：

1. 单行注释

单行注释以两个正斜杠 (//) 开头。该行 // 后面的所有内容都被视为注释，将被 JavaScript 引擎忽略。

语法：

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

2. 多行注释

多行注释以/*开始，以*/结束。 /* 和 */ 之间的所有内容都被视为注释，并且可以跨越多行。

语法：

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

基本输出

显示结果并与用户交互是编程的基础。以下是有关在 JavaScript 环境（例如 Node.js）中执行基本输出操作的综合指南：

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.

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

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

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

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

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

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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