Typescript functions : What does the ":" type" after parameters mean & why some functions don't have it?

Question

The question

In a nutshell :

(It's in the title of the question)

• ":" after function parameters, what does it mean?
• Why is it not always present ?

With examples :

I could see a lot of different ways to write a function in Typescript, for instance :

Ex 1

function buildName(firstName: string, lastName: string) {
  return firstName + " " + lastName;
}

but also,

Ex2

function buildName(firstName: string, lastName: string): string  {
  return firstName + " " + lastName;
}

I couldn't really figure out why if what comes after "function(params):" is a type, why isn't it always present. For example, in Java or C++, a function always tells what type of stuff it returns. When the function returns nothing, this is still indicated in these languages as void.

Answer 1

The Answer

In a nutshell :

• ":" - after function parameters, introduce the type of what's returned by the function.
• not always there? - Typescrict can infer the type of what is returned by the function.

With the previous examples :

Typescript's ": type" after function parameters is there to tell of what type is the stuff return by the function like in Java or C++. However, Typescript by default infers the return type of the function so that it's not always necessary to explicitly write it in the code.

In Ex1 the function buildName returns firstName + " " + lastName which is composed of strings. Hence the compiler knows that the function returns a string.

In Ex2 the function buildName has : string after it's parameters. This means that the programmer explicitely tells that the function returns a string. This can be helpful as the compiler will check that what's returned by the function is really a string or not. If it's not, it will throw an error which can tell the programmer that he did a mistake since the function is not behaving as expected.

Here are some good resources I used to learn functions in Typescript:

Typescript Handbook: functions - The official handbook for typescript.

Typing functions in TypeScript | 2ality - A long but rather complete a well-illustrated guide on typescript's function.

NB : There are similar questions on StackOverflow but not matching what I searched. I decided to do my own.