Declaration vs Expression in JavaScript

In JavaScript, `declaration` and `expression` are fundamental concepts that serve different purposes.

Declaration:

A declaration is a statement that defines a variable, function, or class.

It introduces a new identifier and specifies its type, scope, and initial value (if any). 

Declarations are typically used to define variables, functions, and classes.

Examples of declarations:

// Variable declaration

let x;

// Function declaration

function add(a, b) {

  return a + b;

}

// Class declaration

class Person {

  constructor(name, age) {

    this.name = name;

    this.age = age;

  }

}

Expression:

An expression is a piece of code that evaluates to a value. 

It can be a simple value, a function call, an arithmetic operation, or a more complex expression. 

Expressions are typically used to perform calculations, assign values to variables, or pass arguments to functions.

Examples of expressions:

// Simple value expression

42;

// Function call expression

add(2, 3);

// Arithmetic expression

x = 5 * 2;

// Conditional expression

x > 5 ? 'greater' : 'lesser';

Key differences:

1. Purpose: Declarations introduce new identifiers, while expressions evaluate to values.

2. Syntax: Declarations typically start with a keyword (e.g., `let`, `function`, `class`), while expressions do not.

3. Scope: Declarations have their own scope, while expressions inherit the scope of their surrounding code.

4. Evaluation: Declarations are not evaluated immediately, while expressions are evaluated as soon as they are encountered.

When to use declarations:

1. Defining variables: Use `let`, `const`, or `var` to declare variables.

2. Defining functions: Use the `function` keyword to declare functions.

3. Defining classes: Use the `class` keyword to declare classes.

When to use expressions:

1. Performing calculations: Use expressions to perform arithmetic, logical, or conditional operations.

2. Assigning values: Use expressions to assign values to variables.

3. Passing arguments: Use expressions to pass arguments to functions.

By understanding the differences between declarations and expressions, we can write more effective and efficient JavaScript code.