JavaScript 基礎知識

JavaScript

創建變數

在 JavaScript 中，變數是儲存資料值的容器。本指南將引導您了解如何在 JavaScript 中宣告、初始化和使用變量，以及關鍵規則和概念。

變數宣告和初始化的語法

聲明：
JavaScript 中的變數可以使用 var、let 或 const 來宣告。

初始化：
變數可以在宣告時或之後用一個值進行初始化。

宣告與初始化組合：

使用 const 表示常數：
const 用來宣告一旦初始化就無法重新賦值的變數。

變數宣告：var、let 和 const

• var:聲明一個函數範圍的變量，如果在函數外部聲明，則聲明一個全域範圍的變數。可以重新聲明和更新。

• let:宣告一個區塊作用域變量，這表示它只能在宣告的區塊內存取。它可以更新，但不能在同一範圍內重新聲明。

• const:宣告一個區塊作用域的唯讀變數。它必須在聲明時初始化並且不能重新分配。

變數名稱規則

1. 變數名稱必須以字母、底線 (_) 或美元符號 ($) 開頭。
2. 後續字元可以是字母、數字、底線或美元符號。
3. 變數名稱區分大小寫（myVar 和 myvar 不同）。
4. 變數名稱不能是 JavaScript 中的保留關鍵字（例如 let、function、class）。

範例：

變數的範圍

• 全域作用域：在任何函數或區塊之外宣告的變數都是全域作用域的，可以從程式碼中的任何位置存取。

• 函數作用域：在函數內以 var 宣告的變數是函數作用域的。

• 區塊作用域：在區塊 {} 內用 let 或 const 宣告的變數是區塊作用域的。

吊掛

在 JavaScript 中，變數宣告（但不是初始化）被提升到其作用域的頂端。這意味著變數可以在聲明之前使用。

但是，let 和 const 不允許在聲明之前進行存取（它們處於「臨時死區」）。

常數

JavaScript 中的常數是使用 const 宣告的。設定初始值後就無法重新分配它們。

範例：

但是，如果值是物件或數組，則仍然可以修改其屬性或元素。

預設值

使用 var、let 或 const 宣告但未初始化的變數將具有未定義的值。

範例：

宣告多個變數

您可以在一條語句中宣告多個變量，並用逗號分隔。您也可以在聲明時初始化它們。

範例：

將一個值指派給多個變數

您可以透過連結賦值運算子將相同的值指派給多個變數。

範例：

建立評論

註解是不可執行的語句，有助於描述和解釋程式碼。它們對於使程式碼更具可讀性和可維護性很有用。 JavaScript 支援兩種類型的註解：

1. 單行註釋

單行註解以兩個正斜線 (//) 開頭。該行 // 後面的所有內容都被視為註釋，將被 JavaScript 引擎忽略。

文法：

2. 多行註釋

多行註解以/*開始，以*/結束。 /* 和 */ 之間的所有內容都被視為註釋，並且可以跨越多行。

文法：

基本輸出

顯示結果並與使用者互動是程式設計的基礎。以下是有關在 JavaScript 環境（例如 Node.js）中執行基本輸出操作的綜合指南：

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.

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)

以上是JavaScript 基礎知識的詳細內容。更多資訊請關注PHP中文網其他相關文章！