JavaScript Types: A Complete Guide

JavaScript uses dynamic typing, where the type of a variable is automatically determined based on the value it holds. Types are divided into two main categories: primitive (value-based) and object (reference-based).

1. Primitive Types

1.1 Number

Represents numeric values, either integer or decimal.

Example:

    let age = 25; // integer
    let price = 9.99; // decimal

Special values:

• NaN (Not a Number): Occurs during invalid mathematical operations.
• Infinity and -Infinity: Represent infinity.

Read more here

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1.2 String

Textual data enclosed in single (''), double ("") quotes, or backticks (``).

Example:

    let name = "Anna";
    let message = `Hello, ${name}!`; // template string

Read more here

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1.3 Boolean

Has two values: true (truthy) and false (falsy).

Example:

    let isOnline = true;
    let hasPermission = false;

Read more here

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1.4 Null

Represents an empty or unknown value. Must be explicitly assigned.

Example:

    let user = null; // user has no value

Read more here

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1.5 Undefined

Indicates a variable is declared but not assigned a value.

Example:

    let score;
    console.log(score); // undefined

Read more here

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1.6 Symbol (ES6+)

A unique value used for object property keys.

Example:

    const id = Symbol("id");

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1.7 BigInt (ES2020+)

Represents large integers by appending n.

Example:

    const bigNumber = 123456789012345678901234567890n;

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2. Object Types

Objects store complex data structures and are referenced by their memory location.

2.1 Object

A collection of key-value pairs.

Example:

    let person = {
      name: "Karen",
      age: 30
    };

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2.2 Array

An ordered collection of elements accessible by index.

Example:

    let colors = ["red", "blue", "green"];

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2.3 Function

A reusable block of code that can be invoked.

Example:

    function add(a, b) {
      return a + b;
    }

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2.4 Other Objects

• Date: Handles dates and times.
• RegExp: Regular expressions.
• Error: Error handling.

Many other object types exist.

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3. Type Checking

typeof

The typeof operator returns the type as a string.

    console.log(typeof 42); // "number"
    console.log(typeof "Hello"); // "string"
    console.log(typeof undefined); // "undefined"

instanceof

instanceof checks if an object belongs to a specific class.

    console.log([] instanceof Array); // true

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4. Type Coercion

JavaScript automatically converts types during operations:

• Number → String: 5 + "" → "5"
• String → Number: +"10" → 10
• Boolean conversion: !!0 → false, !!"text" → true

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5. Memory Management

JavaScript manages memory differently based on types:

5.1 Primitive Types (Value-Based)

• Stored in the stack as independent values.
• Each variable has its own copy.

    let a = 10;
    let b = a; // Copies the value
    a = 20;
    console.log(b); // 10 (unchanged)

5.2 Object Types (Reference-Based)

• Stored in the heap with memory references.
• Variables store memory addresses.

    let obj1 = { name: "Anna" };
    let obj2 = obj1; // Copies the reference
    obj1.name = "Karen";
    console.log(obj2.name); // "Karen" (modified)

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6. Best Practices

• Use === (strict equality) instead of == to avoid type coercion.
• Check for NaN with isNaN().
• Declare variables using let and const.