6. Object Types

Apart from primitives, the most common sort of type you'll encounter is an object type. This refers to any JavaScript value with properties, which is almost all of them! To define an object type, we simply list its properties and their types.

For example, here's a function that takes a point-like object:

// The parameter's type annotation is an object type
function printCoord(pt: { x: number; y: number }) {
  console.log("The coordinate's x value is " + pt.x);
  console.log("The coordinate's y value is " + pt.y);
}
printCoord({ x: 3, y: 7 });

Here, we annotated the parameter with a type with two properties - x and y - which are both of type number. You can use , or ; to separate the properties, and the last separator is optional either way.

The type part of each property is also optional. If you don't specify a type, it will be assumed to be any.

Optional Properties

Object types can also specify that some or all of their properties are optional. To do this, add a ? after the property name:

function printName(obj: { first: string; last?: string }) {
  // ...
}
// Both OK
printName({ first: "Bob" });
printName({ first: "Alice", last: "Alisson" });

In JavaScript, if you access a property that doesn't exist, you'll get the value undefined rather than a runtime error. Because of this, when you read from an optional property, you'll have to check for undefined before using it.

function printName(obj: { first: string; last?: string }) {
  // Error - might crash if 'obj.last' wasn't provided!
  console.log(obj.last.toUpperCase());
'obj.last' is possibly 'undefined'.
  if (obj.last !== undefined) {
    // OK
    console.log(obj.last.toUpperCase());
  }
 
  // A safe alternative using modern JavaScript syntax:
  console.log(obj.last?.toUpperCase());
}

— The TypeScript Handbook: Everyday Types

Exercises