Interfaces

One of TypeScript’s core principles is that type checking focuses on the shape that values have. This is sometimes called “duck typing” or “structural subtyping”. In TypeScript, interfaces fill the role of naming these types, and are a powerful way of defining contracts within your code as well as contracts with code outside of your project.

Our First Interface

The easiest way to see how interfaces work is to start with a simple example:

function printLabel(labeledObj: { label: string }) {
  console.log(labeledObj.label);
}

let myObj = { size: 10, label: "Size 10 Object" };
printLabel(myObj);Try

The type checker checks the call to printLabel. The printLabel function has a single parameter that requires that the object passed in has a property called label of type string. Notice that our object actually has more properties than this, but the compiler only checks that at least the ones required are present and match the types required. There are some cases where TypeScript isn’t as lenient, which we’ll cover in a bit.

We can write the same example again, this time using an interface to describe the requirement of having the label property that is a string:

interface LabeledValue {
  label: string;
}

function printLabel(labeledObj: LabeledValue) {
  console.log(labeledObj.label);
}

let myObj = { size: 10, label: "Size 10 Object" };
printLabel(myObj);Try

The interface LabeledValue is a name we can now use to describe the requirement in the previous example. It still represents having a single property called label that is of type string. Notice we didn’t have to explicitly say that the object we pass to printLabel implements this interface like we might have to in other languages. Here, it’s only the shape that matters. If the object we pass to the function meets the requirements listed, then it’s allowed.

It’s worth pointing out that the type checker does not require that these properties come in any sort of order, only that the properties the interface requires are present and have the required type.

Optional Properties

Not all properties of an interface may be required. Some exist under certain conditions or may not be there at all. These optional properties are popular when creating patterns like “option bags” where you pass an object to a function that only has a couple of properties filled in.

Here’s an example of this pattern:

interface SquareConfig {
  color?: string;
  width?: number;
}

function createSquare(config: SquareConfig): { color: string; area: number } {
  let newSquare = { color: "white", area: 100 };
  if (config.color) {
    newSquare.color = config.color;
  }
  if (config.width) {
    newSquare.area = config.width * config.width;
  }
  return newSquare;
}

let mySquare = createSquare({ color: "black" });