Functions basics

• Function

  • input (arguments)
  • code
  • output (return value)
• You provide an argument (aka input but it is called argument(s))

Pass and argument and name the argument value

let square = function(num) {
  console.log(num);
};

square(3); // 3
square(133); // 133

Return something from a function

• Use the return reserved keyword in JavaScript
• note It can only be used a single time in your function

Here is a function that does these 3 things:

1. Has input (arguments)
2. Has code
3. Has output (returns a value)

function-101.js

let square = function(num) {
  let result = num * num;
  return result;
};

square(3);

Houston we have a problem!

• When I run:

$ node function-101.js

• I don’t get anything in the Terminal. What did I do wrong?

  • You didn’t do anything wrong
  • It’s just that the return value isn’t being used

How to show a returned value

• You need to store the returned value inside a variable then log the variable
• We are “referencing” the function by name (using a variable)

let square = function(num) {
  let result = num * num;
  return result;
};

let value = square(3);
let value = square(10);
console.log(value); // 9
console.log(value); // 100

• Now when you run:

$ node function-101.js

• You will see 9 (the returned value) in the Terminal

Try

• Create a function convertFahrenheitToCelsius
• It will take in as an argument the temp in Fahrenheit
• It will run the conversion algorithm
• It will return the temperature in Celsius

• Call the function a couple of times

  • use 32 as argument and it should return 0
  • use 68 as argument and it should return 20
• Print the converted values

function convertFahrenheitToCelsius(temp) {
  // code
  let tempAsCelcius = ((temp - 32) * 5) / 9;

  // return value
  return tempAsCelcius;
}

// reference values
const temp1 = convertFahrenheitToCelsius(32);
const temp2 = convertFahrenheitToCelsius(68);

// output
console.log(temp1); // 0
console.log(temp2); // 20