Type Inference

One common misconception about TypeScript's types is that code needs to explicitly describe types at every possible opportunity. Fortunately this is not the case. TypeScript has a rich type inference system that will "fill in the blanks" for the programmer. Consider the following:

type-inference-finds-error.ts

let numbers = [2, 3, 5, 7, 11];
numbers = ['this will generate a type error'];

tsc ./type-inference-finds-error.ts 
type-inference-finds-error.ts(2,1): error TS2322: Type 'string[]' is not assignable to type 'number[]'.
  Type 'string' is not assignable to type 'number'.

The code contains no extra type information. In fact, it's valid ES6. If var had been used, it would be valid ES5. Yet TypeScript is still able to determine type information.

Type inference can also work through context, which is handy with callbacks. Consider the following:

type-inference-finds-error-2.ts

interface FakeEvent {
  type: string;
}

interface FakeEventHandler {
  (e: FakeEvent): void; 
}

class FakeWindow {
  onMouseDown: FakeEventHandler
}
const fakeWindow = new FakeWindow();

fakeWindow.onMouseDown = (a: number) => {
  // this will fail
};

tsc ./type-inference-finds-error-2.ts 
type-inference-finds-error-2.ts(14,1): error TS2322: Type '(a: number) => void' is not assignable to type 'FakeEventHandler'.
  Types of parameters 'a' and 'e' are incompatible.
    Type 'number' is not assignable to type 'FakeEvent'.
      Property 'type' is missing in type 'Number'.

In this example the context is not obvious since the interfaces have been defined explicitly. In a browser environment with a real window object, this would be a handy feature, especially the type completion of the Event object.