Jan 28, 2026

Lecture Notes

Slides: Functional JavaScript

Functions as Values

• Functions are first-class values in JavaScript — like arrays or numbers
• Can be assigned to variables, passed as parameters, stored in objects
• Parentheses () execute the function; without them, you're passing the function itself

Function Declarations vs Expressions

• Declaration: function greet(name) { return "Hello " + name; }
• Expression: const greet = function(name) { return "Hello " + name; };
• Anonymous functions have no name — used when passing functions as values

Arrow Functions

• Shorter syntax: const greet = (name) => { return "Hello " + name; };
• Implicit return: const greet = (name) => "Hello " + name;
• Single parameter doesn't need parentheses: const double = x => x * 2;

Callbacks

• A callback is a function passed to another function to be executed later
• Don't use () when passing — that calls it immediately
• Example: setTimeout(sayHello, 1000) — passes the function, doesn't call it

Array Iteration Methods

• forEach(callback): Executes callback on each element (no return value)
• map(callback): Returns new array with transformed values
• filter(callback): Returns new array with elements where callback returns true
• reduce(callback, initial): Reduces array to single value

Callback Parameters

• Array methods pass up to 3 arguments: (item, index, array)
• You can use any or none of them
• Example: nums.map((num, i) => num * i)