Day Datatypes, Variables,Operators, Conditional Statements,Looping in Javascript

JavaScript data type:

In JavaScript, data types define the types of values ​​that a variable can hold. The type determines what operations can be performed on the value. JavaScript has both primitive data types (simple values) and reference data types (complex objects).

1. Original data type:

These are the most basic data types in JavaScript, they hold a single value. They are immutable, which means that once their values ​​are set, they cannot be changed.

a) Number:

represents integers and floating point numbers.

let x = 42;

b) String:

represents a series of characters.

let name = "Alice";

c) Boolean:

means true or false.

let isActive = true;

d) Null:

means intentionally not having any object value.

let person = null;

e) Undefined:

Indicates a variable that has been declared but not assigned a value.

let value;

f) Symbol:

A unique identifier.

const uniqueID = Symbol();

g) BigInt:

is used to represent large integers.

let bigNum = 123n;

2. Reference data type:

These data types are more complex, storing collections of values ​​or more structured data. They are mutable, which means their contents can change even if the reference remains the same.

a) Object:

A collection of key-value pairs.

let person = {name: "John"};

b) Array:

A sorted collection of values.

let arr = [1, 2, 3];

c) Function:

Code block that performs tasks.

function greet() {}

JavaScript variables:

In JavaScript, variables are named containers for storing data that can be referenced and manipulated throughout your code.

Variables allow you to store values ​​such as numbers, strings, objects, and functions.

1. Declare variables:

In JavaScript, variables are declared using one of three keywords:

var (old method, less used now) let (block scope, introduced in ES6) const (block scope, for constants, also introduced in ES6)

a) var (old method):

The var keyword is traditionally used to declare variables in JavaScript.

var name = "Alice"; var age = 25;

b) let (modern method):

Block scope means that a variable can only be accessed within the block in which it is defined (for example, within an if statement or loop).

let name = "Bob"; let age = 30;

c) const (constant variable):

The const keyword is used to declare variables that should not be reassigned after initial assignment.

const country = "USA";

Redeclare variables:

Use var: You can redeclare variables within the same scope and JavaScript will not throw an error.

<code class="language-javascript">var name = "Alice";
var name = "Bob";  // 没有错误
console.log(name);  // 输出：Bob</code>

Use let and const: You cannot redeclare a variable within the same scope. Doing so will cause a SyntaxError.

<code class="language-javascript">let name = "Alice";
let name = "Bob";  // SyntaxError: Identifier 'name' has already been declared</code>

<code class="language-javascript">const country = "USA";
const country = "Canada";  // SyntaxError: Identifier 'country' has already been declared</code>

JavaScript operators:

In JavaScript, operators are symbols or keywords that perform operations on operands.

1. Arithmetic operators:

These operators are used to perform mathematical operations on numerical values.

<code>+               加法                        5 + 3 → 8
-               减法                        5 - 3 → 2
*               乘法                        5 * 3 → 15
/               除法                        5 / 3 → 1.6667
%               取模（除法的余数）        5 % 3 → 2
**              幂运算（乘方）              2 ** 3 → 8
++              递增（加 1）                let x = 5; x++ → 6
--              递减（减 1）                let x = 5; x-- → 4</code>

2. Assignment operator:

These operators are used to assign values ​​to variables.

<code>=               简单赋值                     let x = 10;
+=              加法赋值                     x += 5; → x = x + 5;
-=              减法赋值                     x -= 5; → x = x - 5;
*=              乘法赋值                     x *= 5; → x = x * 5;
/=              除法赋值                     x /= 5; → x = x / 5;
%=              取模赋值                     x %= 5; → x = x % 5;
**=             幂赋值                       x **= 2; → x = x ** 2;</code>

3. Comparison operators:

These operators compare two values ​​and return a Boolean value (true or false).

<code>==              等于                          5 == "5" → true
===             全等                          5 === "5" → false
!=              不等于                        5 != "5" → false
!==             不全等                        5 !== "5" → true
>               大于                          5 > 3 → true
>=              大于等于                      5 >= 3 → true
<               小于                          5 < 3 → false
<=              小于等于                      5 <= 3 → false</code>

4. Logical operators:

These operators are used to perform logical operations, usually using Boolean values.

<code>&&              逻辑与                        true && false → false
||              逻辑或                        true || false → true
!               逻辑非                        !true → false</code>

JavaScript conditional statement:

In JavaScript, conditional statements are used to perform different actions based on whether a specific condition is true or false.

1. if statement

The if statement is the most basic conditional statement. If the specified condition evaluates to true, the code block is executed.

Grammar:

<code class="language-javascript">if (condition) {
    // 如果条件为真，则执行的代码块
}</code>

2. else statement

The

else statement follows the if statement and defines a block of code that is executed if the condition in the if statement is false.

Grammar:

<code class="language-javascript">if (condition) {
    // 如果条件为真，则执行的代码块
} else {
    // 如果条件为假，则执行的代码块
}</code>

3. else if statement

The else if statement allows you to test multiple conditions. Use it when you need to check multiple conditions.

Grammar:

<code class="language-javascript">if (condition1) {
    // 如果 condition1 为真，则执行的代码块
} else if (condition2) {
    // 如果 condition2 为真，则执行的代码块
} else {
    // 如果 condition1 和 condition2 都为假，则执行的代码块
}</code>

4. Nested if statements

You can nest if statements within other if statements to create more complex conditions.

5. switch statement

The switch statement is a more efficient way to test a value with multiple conditions, especially when you have many different possible outcomes.

Grammar:

<code class="language-javascript">switch (expression) {
    case value1:
        // 如果 expression === value1，则运行的代码
        break;
    case value2:
        // 如果 expression === value2，则运行的代码
        break;
    default:
        // 如果没有 case 匹配，则运行的代码
}</code>

JavaScript loop:

In JavaScript, a loop is used to repeatedly execute a block of code as long as a specific condition is met. This is useful when you want to perform repetitive tasks, such as iterating over an array or executing a block of code multiple times.

1. for loop

The for loop is the most common loop in JavaScript. This is typically used when you know how many times you want to iterate a block of code.

Grammar:

<code class="language-javascript">for (initialization; condition; increment/decrement) {
    // 每次迭代中要执行的代码
}</code>

2. while loop

The while loop executes a block of code as long as the specified condition evaluates to true. The condition is checked before each iteration and the loop stops once the condition becomes false.

Grammar:

<code class="language-javascript">while (condition) {
    // 只要条件为真就执行的代码
}</code>

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