Comp 110 Functions

Defining a function

function <name> (<parameter name: type>, ...): <return type> {
     <function body>

     return <expression>;


  • located within the () of a function definition
  • refer to variables that the user passes in when the function is called
  • are local to the function definition (can only be used inside the { } of the function definition)

Return Type

  • can be numberbooleanstring, or void
  • function must return an object or expression that simplifies to the same type as the return type
  • void functions do not contain a return statement

Function Body

  • located within the { } of the function
  • gives 'functionality' to the function
  • code will only be executed when the function is called elsewhere in the code
  • if not void, MUST contain a return statement

Return Statement

  • type must match the return type of its function
  • every function that returns a value must have at least one return statement
  • as soon as any return statement is reached the function call is complete
    • even if within an if-then-else block!

Example function:

let matchMaker = (personOne: string, personTwo: string): string => {

    let couple: "Congratulations" + personOne + " " + personTwo;

   return couple;


We can see the parts of the function in the following way:

Parameters: PersonOne and PersonTwo (both strings)

Return type: string

Function body: let couple: "Congratulations" + personOne + " " + personTwo;

Return statement: return couple;

Although in the example above, we only have one type in our parameters, you can have multiple

For example, we could modify the matchMaker function in the following way:

let matchMaker = (personOne: string, personTwo: string, areCompatable: boolean): string => {

  let couple: "I'm not sure...";

   if (areCompatable) {
     let couple: "Congratulations " + personOne + " and " + personTwo;

   return couple;


If we wanted to use the function, we would just call it using the function name followed by ( ) where we would include arguments to match the parameters the function requires.

Arguments are what we pass into parameters. If you are fuzzy on the difference between arguments and parameters, read here!

For example, calling matchMaker as it appears in the example above:

let result: matchMaker("Jim", "Pam", true);

// would return "Congratulations Jim and Pam" which would become result's value

Arguments don't have to be literals though, we can also (and much more commonly) pass variables into functions.

For example in the simple findSum function below:

let findSum = (x: number, y: number): number => {
     let sum: number;
     sum = x + y;
     return sum;

A helpful way to think of a function is to compare it to a recipe:

  • parameters are the ingredients we need (can't make the recipe without the correct quantity and type of ingredients!)
  • the function body is the instructions
  • the return type would be whatever we want to make!