Functions 🔥

Functions are reusable blocks of code that perform a specific task.

• Purpose:
  The function keyword is used to define functions in JavaScript. Functions are fundamental building blocks that encapsulate code for reuse and modularity.

• Functions Are First-Class Objects:
  Functions can be assigned to variables, passed as arguments, and returned from other functions. This makes them very flexible in JavaScript.

Function Declarations

Note

Function declarations are hoisted to the top of their scope. This means you can call the function before its declaration in the code.

function functionName(parameters) {}

Function Expressions

Note

Unlike function declarations, function expressions are not hoisted. The function is only available after its definition.

// Anonymous function expression (no internal name)
const anonymousFunc = function (params) {};

// Named function expression (internal name useful for recursion & debugging)
const recursiveFunc = function innerRecursive(params) {};

// Arrow function expression (concise syntax from ES6)
const arrowFunc = (params) => {};

// Oneline Arrow function expression (concise syntax from ES6)
const arrowFunc = (params) => line - code;

1. Safe Approach (Using Named Function Expression)

let fact = function _fact(n) {
  if (n == 0) return 1;
  return n * _fact(n - 1); // Always refers to the internal function
};

const newfact = fact;
fact = null;
console.log(newfact(5)); //Ouptput: 120

2. Risky Approach (Using fact for Recursion)

let fact = function _fact(n) {
  if (n == 0) return 1;
  return n * fact(n - 1); // Uses fact instead of _fact
};

const newfact = fact;
fact = null;
console.log(newfact(5)); // ❌ TypeError: fact is not a function

Arguments & Parameters

info

• Arguments: Arguments are values passed to a function when it is called.
• Parameters: Parameters allow you to initialize function parameters

• Default Parameters

  • Allow parameters to have fallback values if no argument (or undefined) is provided.
  • Syntax Example:

    function multiply(a = 1, b = 1) {
      return a * b;
    }
    console.log(multiply(5)); // 5, because b defaults to 1

  • Evaluated at call time in a left-to-right order. Earlier parameters can be used in later default expressions.

• Rest Parameters

  • Gather all remaining arguments (beyond the named parameters) into a real Array.
  • Syntax Example:

    function sum(a, b, ...others) {
      return a + b + others.reduce((acc, x) => acc + x, 0);
    }
    console.log(sum(1, 2, 3, 4)); // 10

  • Restrictions
    • Only one rest parameter is allowed per function.
    • It must be the last parameter.
    • Trailing commas after it are not allowed.
    • It cannot have a default value.