JavaScript, difference between String and string

What is difference between String and string in Javascript?

In JavaScript, there is a subtle but important difference between String (with an uppercase "S") and string (with a lowercase "s"). Here's the distinction:

1. String (with an uppercase "S"):

String is a constructor function and a built-in object in JavaScript.

You can use String to create a new string object or to call static methods such as String.prototype methods.

String is also used to convert other data types to a string (using String() function).

It creates objects that are instances of the String class, which means they can have methods available through the String.prototype.

Example:

let strObject = new String('Hello, world!');

console.log(strObject instanceof String); // true

In this example, strObject is an object that wraps the string 'Hello, world!'. This object has methods defined on String.prototype, like .toUpperCase(), .slice(), etc.

2. string (with a lowercase "s"):

string refers to the primitive data type for representing text.

In JavaScript, strings are immutable and are primitive values, not objects.

When you use a string directly (e.g., 'Hello'), you're working with a string primitive, not an object. JavaScript automatically wraps these primitives with String objects when needed (e.g., to access string methods), but the value remains a primitive string.

Example:

let strPrimitive = 'Hello, world!';

console.log(strPrimitive instanceof String); // false

console.log(strPrimitive.toUpperCase());    // 'HELLO, WORLD!'

In this example, strPrimitive is a primitive string. Even though it's a primitive, JavaScript allows us to call methods like .toUpperCase() on it by temporarily converting it to a String object.

Key Differences:

Feature String (uppercase "S") string (lowercase "s")

Type A constructor function (also a global object). A primitive data type (not an object).

Creation Created using new String(). Created directly using a string literal ('...').

Behavior Represents an object that wraps a string. Represents a simple string value.

Methods Has methods available on String.prototype. Methods are available through automatic boxing (temporary object creation).

Memory Requires more memory because it is an object. Requires less memory as it's a primitive.

Summary:

String (uppercase) is an object constructor, used to create string objects with methods that are part of String.prototype.

string (lowercase) is a primitive data type used to represent textual data. In most cases, you'll use strings as primitives, and JavaScript will automatically give you access to string methods without explicitly creating an object.