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Basic Syntax and Variables

 

Understanding JavaScript’s basic syntax and how to work with variables is essential for writing effective code.

Statements and Semicolons

  • A statement is a single instruction that tells the browser to do something.
  • In JavaScript, statements typically end with a semicolon (;).
  • Although semicolons are optional in many cases due to Automatic Semicolon Insertion (ASI), it’s good practice to include them to avoid unexpected errors.

Example:

let message = "Hello, world!";

console.log(message);

Comments

Comments help you document your code and make it easier to understand. JavaScript supports two types:

  • Single-line comments start with //

// This is a single-line comment

  • Multi-line comments are wrapped between /* and */

/*

  This is a

  multi-line comment

*/

Variables: var, let, and const

Variables store data values. JavaScript provides three keywords to declare variables:

  • var
    The old way to declare variables. Function-scoped and allows redeclaration. Generally avoided in modern code.
  • let
    Block-scoped and allows reassignment but not redeclaration in the same scope. Preferred for variables that change.
  • const
    Block-scoped and cannot be reassigned after initial assignment. Used for constants or values that shouldn’t change.

Example:

var age = 25;

let name = "Alice";

const PI = 3.14159;

Data Types

JavaScript variables can hold different types of data:

  • String: Text enclosed in quotes
    "Hello", 'World'
  • Number: Numeric values (integers or decimals)
    42, 3.14
  • Boolean: Logical values true or false
  • Null: Represents an intentional absence of any value
    let data = null;
  • Undefined: A variable declared but not assigned a value yet
    let result; // undefined
  • Symbol: A unique and immutable primitive value used for identifiers
    let id = Symbol('id');

 

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