What is this ES6 syntax? Colon after function call [duplicate]

Question

I was looking at the docs for a flux store in React. They gave the following example.

import {ReduceStore} from 'flux/utils';

class CounterStore extends ReduceStore<number> {
  getInitialState(): number {
    return 0;
  }

  reduce(state: number, action: Object): number {
    switch (action.type) {
      case 'increment':
        return state + 1;

      case 'square':
        return state * state;

      default:
        return state;
    }
  }
}

See getInitialState(): number {}, this doesn't seem to follow any previous javascript syntax convention. How would one write this using ES5 syntax?

Answer 1

That is Typescript and it is annotating what type of value the function returns. getInitialState returns a number.

You can see the same annotation in the arguments being passed to reduce(). This sort of annotation lets your IDE/text editor do some really helpful suggestions and error checking.

Answer 2

This is not part of ES6 syntax. You're looking at type definitions for static type checkers, such as flowjs or TypeScript. There is no ES5 syntax for that.