class vs type in Flow

Question

In Flow, why would one use a class versus a type?

type Point = {x: number; y: number};
class Point = {x: number; y: number};

Answer 1

In your example, a type is all you need.

But if you want to define methods, you'll want to use a class.

class Point {
    x: number;
    y: number;
    constructor(x, y) {this.x = x; this.y = y;}
    distance_from_origin(): number { /* code goes here */ }
    angle_from_origin(): number { /* code goes here */ }
}

p: Point = new Point(2, 3);
d = p.distance_from_origin()

Types are a flow features for compile-time checking, to help you catch errors in your code. They are entirely stripped from the code before running it.

Classes aren't a Flow feature at all (Although Flow understands classes - every class you create also defines a Flow type) - they're a feature of ES6, the next version of JavaScript. Babel, the same program that strips Flow types from your code to make it valid JavaScript, can also convert your classes to ES5-compatible code.