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In TypeScript, function parameters may be given type annotations with the same syntax as variable declarations: a colon next to the name. The type annotations ensure that the parameters are of the correct type:

function greet(name: string) {
  console.log(`Hello, ${name}!`);
}

greet('Katz'); // Prints: Hello, Katz  

greet(1337); // Error: argument '1337' is not assignable to parameter of type 'string'

By placing : string after the name parameter, we’re declaring that name is of type string. Any invocation of greet() should pass a value of type string as the first argument, or else an error will be thrown.

Parameters that we do not provide type annotations for are assumed to be of type any—the same way variables are.

function printKeyValue(key: string, value) {
  console.log(`${key}: ${value}`);
}

printKeyValue('Courage', 1337); // Prints: Courage: 1337
printKeyValue('Mood', 'scared'); // Prints: Mood: scared

Here, the parameter value is an any variable: it’s compatible with any type.

Instructions

The triple() function is meant to multiply a value by three. Add a type annotation to the value parameter to indicate that it’s meant to be a number.

greetTripled() is meant to take in a string and a number. Add type annotations to its type parameters as well.

Everything seems to look good. Let’s quickly compile our code and move on to the next exercise. To do this, simply run tsc in the terminal.

Woah! TypeScript found a bug, thanks to our type annotations. It looks like greetTripled() is being called with the wrong types of arguments. Fix the code to print Hiya, 15!.

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