Functions

Lesson objectives

After this lesson students will be able to:

1. Describe why we use functions
2. Declare a function
3. Invoke a function
4. Properly name a function
5. Write an arrow function with a parameter
6. Write an arrow function with multiple parameters
7. Write an arrow function with a return statement
8. Break a problem down into steps

Setup

In student_examples for today, make a file functions.js

Test that a console.log will appear in Terminal when you run the file.

node functions.js

Describe why we use functions

Using functions is another application of DRY. Don't Repeat Yourself. With a function, you can store code that can be used conveniently as many times as you wish, without having to rewrite the code each time.

Defining a function

/** @type {() => void}  */
const printBoo = () => {
  console.log("======");
  console.log("Boo!");
  console.log("======");
};

Always use const to declare your functions. It would be a strange day when a function would need to be reassigned.

note

The code will not run yet. The function needs to be invoked.

Invoke a function

Use one line of code to run multiple lines of code

printBoo();

Simply use the name of the variable and use parentheses to invoke the function.

caution

If the parentheses are not included, the function will not run.

The invocation comes after the function definition. If you write it beforehand, it will be trying to invoke something that doesn't yet exist according to the interpreter.

This will work:

const printBoo = () => {
  console.log("======");
  console.log("Boo!");
  console.log("======");
};

printBoo();

VS

This will not:

printBoo();

const printBoo = () => {
 console.log('======');
 console.log('Boo!');
 console.log('======');
};

Exercise

• Write a function printSum that will console.log the result of 10 + 10

Extra Reps

• Write a function printTriangle that will print these pound signs to the console (there are 5 console.logs inside the function):

  #
  ##
  ###
  ####
  #####

• Make it so that printTriangle will print the pound signs using a for loop (there is a for loop and only 1 console.log inside the function).

• Make it so that when you can invoke the function with the number of pound signs to print (not just hardcoded to print 5)

Properly name a function

The variable you use for a function should contain a verb. Functions do something, most often:

• getting data
• setting data
• checking data
• printing data

If the purpose of your function is to check data, for example, use the verb check in the variable name.

Example function that contains a conditional:

/** @type {(input: string | array) => void}  */
const checkInputLength = (input) => {
  if (input.length > 10) {
    console.log("input length is greater than 10");
  } else {
    console.log("input length is not greater than 10");
  }
};

Functions should try to do only one thing

If a function, called checkInputLength, does more than just check input, then it is a poor function.

Takeaway: Think about appropriate verbs to use in your function variable names. The verbs should indicate the one thing that the function does.

Write an arrow function with a parameter

The preceding function, checkInputLength had a parameter called input.

Functions can receive input that modify their behavior a bit. This input is called a parameter.

In the below example, the parameter is arbitrarily called name. We can call our parameters whatever we want - whatever makes semantic sense.

Using concatenation I can put the input into a string:

/** @type {(name: string) => void}  */
const sayName = (name) => {
  console.log("Hello! My name is " + name);
};

When we invoke the function, we can specify the value of the parameter, this is called an argument:

sayName("Frodo");

We can continue to invoke the function with whatever arguments we want:

sayName("Merry");
sayName("Pippin");
sayName("Sam");

Each time, the output of the function will change to reflect the argument.

Argument vs Parameter

The argument is the input, the parameter is how the input is represented in the function.

const func = (PARAMETER) => {
 // some code
}

func(ARGUMENT);

Write an arrow function with multiple parameters

A function can take any number of parameters.

/** @type {(num1: number, num2: number) => void}  */
const calculateArea = (num1, num2) => {
  console.log(num1 * num2);
};

When you invoke the function, you generally want to supply the right number of arguments.

calculateArea(4, 4)

=> 16

Exercises

• Write a function called minusOne that takes a parameter num. Assuming the argument is a number, print the argument -1.

  minusOne(10);        // 9
  minusOne(100);       // 99
  minusOne(Infinity);  // Infinity

• Write a function makeSentence that takes three parameters and concatenates them into a fully formed sentence.

makeSentence('I', 'want', 'chimichangas');

=> 'Oh boy, do I want chimichangas or what?'

Extra

• Write a function called getLastElement that takes a parameter arr.
• Invoke the function with an array as the argument.
• The function should print the last element within the array.

getLastElement([1, 2, 3, 4, 5, 6]);       // 6
getLastElement(['a', 'b', 'c']);          // 'c'
getLastElement([[1, 2, 3], [4, 5, 6]]);   // [4, 5, 6]

Hint: arr[arr.length - 1]

Extra II

• Write a function divideThreeNums that takes three parameters and prints the third parameter divided by the result of the second parameter divided by the first.

divideThreeNums(10, 5, 2)   // 4
divideThreeNums(30, 2, 9)   // 135

Write an arrow function with a return statement

Functions can pass information back to whatever line of code invoked them.

A contrived example:

/** @type {() => number}  */
const ten = () => {
  return 10;
};

console.log(8 + ten());

=> 18

Here ten() gets replaced with whatever the return statement is.

This is the main difference between return and console.log(). The output value of a function can not come from a console.log.

/** @type {() => void}  */
const ten = () => {
 console.log(10);
}
console.log(8 + ten());

=> 10 => NaN

This is because it is trying to add 8 to undefined.

A function is only defined if it has a return value.

/** @type {(num1: number, num2: number) => number}  */
const calculateArea = (num1, num2) => {
  return num1 * num2;
};

The calculateArea function has a return value of num1 * num2. It does not just print to the console.

use the returned value of an invoked function

Sometimes a returned value will not appear in your console. This is normal. A return is not a console.log. To see the return value of a function, you will want to console.log the invocation:

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

=> 20

Since calculateArea returns a value, we can use the return value of calculateArea as an argument to an invocation of calculateArea.

console.log(calculateArea(calculateArea(2, 3), calculateArea(9, 4)));

=> 216

return: stopping a function

RETURN sends the value of your function immediately. You can use return to terminate the function.

Example: will the function return 0 or 1? (It won't return both)

/** @type {(input: string) => number} */
const example = (input) => {
  if (input === "none") {
    return 0;
  }
  return 1; // if input is "none" this line will not run
};

example("none")     // 0
example("two");     // 1

Break a problem down into steps

Write a function that will return true if a word is a Palindrome, or will return false if not.

• Problem solve one step at a time
• Each step might require research

Work in layers, one layer at a time. Don't jump ahead until each piece has been tested and works.

Determine if each step will require research, and research it.

• reverse the word (how?)
• check if the word is the same as the reverse (how?)
• return true or false

Exercises II

• Write a function that takes two parameters (strings) and returns true (Boolean) if the two strings are identical, false if not.

• Write a function that takes three parameters (numbers), sums them, converts the sum into a string and returns the string (eg. "123")

  • Use your google-fu to research converting a number into a string
  • Invoke the function a couple of times with different arguments each time

• Experiment

  • What happens if you supply more arguments than there are parameters?
  • What happens if you supply fewer arguments than there are parameters?