Name a Reusable Task

A function accepts inputs through parameters and gives a result back with return. A pass rule can be reused by different reports without duplicating its threshold. Returning a value is different from printing it.

Before this topic, review Make Strict-Mode Errors Visible if its terms are unfamiliar.

This is JavaScript using language features and the host console. Read the initial values, then follow each operation to its displayed result. A declaration names a value, a function call performs an operation, and console.log displays its arguments.

In JavaScript, functions are reusable blocks of code designed to perform a specific task. They're the building blocks of any application – small or large. Functions help break complex problems into manageable parts and make your code modular, readable, and reusable.

You can define a function using the function keyword followed by a name, parentheses (), and a block of code inside curly braces {}.

function greet() {
  console.log("Hello, welcome to JavaScript!");
}

After defining the function, you must call or invoke it using its name followed by parentheses.

function greet() {
  console.log("Hello, welcome to JavaScript!");
}

greet();

Hello, welcome to JavaScript!

You can pass data into functions through parameters, and use arguments when calling them.

function greetUser(name) {
  console.log("Hello, " + name + "!");
}

greetUser("Akira");

Hello, Akira!

The return keyword sends a value back to the place where the function was called.

function add(a, b) {
  return a + b;
}

let result = add(5, 3);
console.log(result);

8

Functions can be assigned to variables. These are called function expressions.

const multiply = function(a, b) {
  return a * b;
};

console.log(multiply(4, 2));

8

Arrow functions provide a shorter syntax and lexically bind the this value.

const divide = (a, b) => {
  return a / b;
};

console.log(divide(10, 2));

5

5

When there's a single return statement, you can make it more concise:

const square = x => x * x;
console.log(square(6));
console.log(square(10));

36
100

You can assign default values to parameters.

function greet(name = "Guest") {
  console.log("Hello, " + name);
}

greet();       // Uses default
greet("Ravi"); // Uses given argument

Hello, Guest
Hello, Ravi

When you don't know how many arguments you'll receive, use the rest operator ....

function sum(...numbers) {
  return numbers.reduce((acc, curr) => acc + curr, 0);
}

console.log(sum(1, 2, 3, 4));

10

function process(callback) {
  console.log("Processing...");
  callback();
}

function done() {
  console.log("Done!");
}

process(done);

Processing...
Done!

IIFEs are functions that execute immediately after definition.

(function() {
  console.log("IIFE executed!");
})();

IIFE executed!

Functions define their own scope. Variables declared inside a function are local and not accessible outside.

function showScope() {
  let message = "Inside function";
  console.log(message);
}

showScope();
console.log(message); // Error

Inside function
ReferenceError: message is not defined

A closure gives you access to an outer function’s variables even after the outer function has finished executing.

function outer() {
  let count = 0;
  return function inner() {
    count++;
    console.log("Count: " + count);
  };
}

const counter = outer();
counter();
counter();
counter();

Count: 1
Count: 2
Count: 3

Pure functions always return the same output for the same input and have no side effects.

// Pure
function pureAdd(a, b) {
  return a + b;
}

Impure functions may change external state or depend on it.

// Impure
let counter = 0;
function impureAdd() {
  counter++;
  return counter;
}

• Functions are core to writing reusable code in JavaScript.
• There are multiple ways to declare functions: declarations, expressions, and arrow functions.
• They support parameters, return values, rest arguments, and callbacks.
• Closures and scopes are essential for managing variable access and function behavior.

Write a function called greetTime(name, time) that returns:

• "Good morning, Alice" if time = "morning"
• "Good evening, Alice" if time = "evening"
• etc.

This will help you practice if-else and string concatenation inside a function!

A function without an explicit returned value returns undefined. Returning ends that invocation; statements afterward do not execute.

Your task: Replace passed(72) with passed(40). Predict the result before running the edited program.

The source is repeated here. If the edit targets the HTML rather than the script, use the markup repeated below; the worked answer also repeats the changed markup.

The function compares the supplied 40 with the same pass threshold.

A function accepts inputs through parameters and gives a result back with return. A pass rule can be reused by different reports without duplicating its threshold. Returning a value is different from printing it.

Check your variation against this explanation: The function compares the supplied 40 with the same pass threshold.

Use the JavaScript array reference library for individual array operations and boundary tests to check a rule independently.