Chapter 5. Functions

Function arguments

In one end, out the other

As a return type

In Chapter 2, “The Type System”, you saw how to use type annotations to annotate values of variables. Now, you’ll see how to do the same with function parameters and return types—and why that can be useful.

Function Parameters

Take the following sing function that takes in a song parameter and logs it:

function sing(song) {
  console.log(`Singing: ${song}!`);

What value type did the developer who wrote the sing function intend for the song parameter to be provided with?

Is it a string? Is it an object with an overridden toString() method? Is this code buggy? Who knows?!

Without explicit type information declared, we may never know—TypeScript will consider it to be the any type, meaning the parameter’s type could be anything.

As with variables, TypeScript allows you to declare the type of function parameters with a type annotation. Now we can use a : string to tell TypeScript that the song parameter is of type string:

function sing(song: string) {
  console.log(`Singing: ${song}!`);

Much better: now we know what type song is meant to be!

Note that you don’t need to add proper type annotations to function parameters for your code to be valid TypeScript syntax. TypeScript might yell at you with type errors, but the emitted JavaScript will still run. The previous code snippet missing a type declaration on the song parameter will still convert from TypeScript to JavaScript. Chapter 13, “Configuration ...

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