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Functions in JavaScript

Declaring Functions in Different Ways​

Standard Function Declaration​

function name() {}

Function Expression​

var name = function () {};

Anonymous Function​

  • Functions without a name, often used in callbacks.
setTimeout(function () {
  console.log("This is an anonymous function.");
}, 1000);

Named Function Expression​

var name = function funcName() {};

Inline Function with Function Constructor​

var func = new Function("a", "b", "return a + b");
console.log(func(1, 2)); // 3

Function Parameters​

Passing Function Parameters​

  • Parameters are passed to functions as arguments.

Local Scope of Arguments​

  • Arguments passed to a function are local to that function.
function test(a) {
  console.log(a); // Local to the function
}

Pass by Value vs Pass by Reference​

  • Primitive values: Passed by value.
  • Objects: Passed by reference.

Passing Functions as Parameters (Callbacks)​

  • Functions can be passed as arguments.
function greet(callback) {
  callback();
}
greet(function () {
  console.log("Hello!");
});

Function Behavior Based on Number of Arguments​

  • Functions can behave differently based on the number or type of arguments passed.
function sum(a, b) {
  if (b === undefined) return a;
  return a + b;
}

Rest Parameters​

  • Collects all remaining arguments into an array.
function sum(...args) {
  return args.reduce((a, b) => a + b, 0);
}
console.log(sum(1, 2, 3)); // 6

arguments in Functions​

  • Array-like object containing all arguments passed to the function.
function example() {
  console.log(arguments[0]); // First argument
}
example(1, 2); // 1

Breaking Function Execution with return​

  • Functions can be exited early with return.
function test(a) {
  if (a > 10) return;
  console.log("a is less than or equal to 10");
}

Default Parameters​

  • Provides default values for parameters.
function greet(name = "Guest") {
  console.log(`Hello, ${name}!`);
}
greet(); // Hello, Guest!

Immediate Invoking Functions (IIF)​

  • Functions executed immediately after their declaration.
(function () {
  console.log("I am invoked immediately!");
})();

Closures​

  • Functions that remember the scope in which they were created.
function outer() {
  let count = 0;
  return function inner() {
    count++;
    return count;
  };
}
const counter = outer();
console.log(counter()); // 1
console.log(counter()); // 2

Currying​

  • Transforming a function with multiple arguments into a series of functions with a single argument.
function curry(func) {
  return function curried(...args) {
    if (args.length >= func.length) {
      return func(...args);
    } else {
      return (...nextArgs) => curried(...args, ...nextArgs);
    }
  };
}
const sum = (a, b, c) => a + b + c;
const curriedSum = curry(sum);
console.log(curriedSum(1)(2)(3)); // 6

Predefined Methods in JavaScript​

eval()​

  • Evaluates a string as JavaScript code.
    Note: Use with caution due to security concerns.
eval("console.log(2 + 2)"); // 4

isFinite()​

  • Checks if a value is a finite number.
isFinite(10); // true
isFinite(Infinity); // false

isNaN()​

  • Checks if a value is NaN.
isNaN("abc"); // true

parseFloat()​

  • Parses a string and returns a floating-point number.
parseFloat("3.14"); // 3.14

parseInt()​

  • Parses a string and returns an integer.
parseInt("42px", 10); // 42

decodeURI()​

  • Decodes a Uniform Resource Identifier (URI).
decodeURI("https%3A%2F%2Fexample.com"); // "https://example.com"

decodeURIComponent()​

  • Decodes a URI component.
decodeURIComponent("example%20space"); // "example space"

encodeURI()​

  • Encodes a URI, allowing certain characters: A-Z, a-z, 0-9, ; , / ? : @ & = + $ - _ . ! ~ * ' ( ) #
encodeURI("https://example.com?name=John&age=30");

encodeURIComponent()​

  • Encodes a URI component, escaping additional characters such as ; / ? : @ & = + $ , #.
encodeURIComponent("John & Jane"); // "John%20%26%20Jane"

escape()​

  • Encodes a string, but is deprecated.

unescape()​

  • Decodes a string encoded with escape().