Functions

A function is a named, independent section of code that performs a specific task and optionally returns a computed value.

Table of Contents

1. Functions
2. Function Definition and Execution
3. Function Arguments
4. Return Values
5. Variable Scope

Functions

Functions are the verbs of computer programming. They do things.

They increase:

• Modularity, by allowing us to break down large programs into smaller parts.
• Reusability, by allowing us to reuse code without having to retype or copy/paste it.
• Readability, by allowing us to give descriptive names to specific parts of our program.

Function Definition and Execution

Functions are defined in Javascript using the function keyword:

function sayGoodnight() {
  console.log("So Long");
  console.log("Farewell");
}

// To execute this function:
sayGoodnight();

Function Arguments

Functions can be defined to take one or more arguments by way of a parameter list.

Default values for the parameters can also be specified.

function sayGoodnightRepeatedly(name, numberOfTimes = 1) {
  // Default value for numberOfTimes
  for (let i = 0; i < numberOfTimes; i++) {
    console.log(`Goodnight ${name}`);
  }
}

sayGoodnightRepeatedly("Wally", 12);
sayGoodnightRepeatedly("Wally"); // Second argument defaults to 1.

Return Values

Functions can optionally return a value to the caller.

function pizzaMessage(piecesLeft, hungryPeople) {
  if (piecesLeft < hungryPeople) {
    return "Sorry we don't have enough pizza.";
  } else {
    return "Let's share! Any leftovers go to the dog.";
  }
}

console.log(pizzaMessage(13, 7));

Variable Scope

Variables defined with let are scoped to the function or the block in which they are defined.

function gemFactory(numberOfGems) {
  let gems = "💎".repeat(numberOfGems); // The gems variable is a local variable.
  return gems;
}

console.log(gemFactory(5));
console.log(gems); // Error. Variable is out of scope.

let requestedGems = Number(prompt("How many gems to you want?")); // Converts the user's string to a number.
if (requestedGems > 0) {
  let yourGems = gemFactory(requestedGems); // yourGems is only scoped to this block.
}

console.log(yourGems); // Error. The yourGems variable is out of scope.