javascript - how to declare that a given class implements an interface in Facebook Flow? -

i have following code using flow:

// @flow     'use strict';  import assert 'assert';  declare interface ipoint {     x: number;     y: number;     distanceto(other: ipoint): number; }  class point {     x: number;     y: number;     distanceto(a: ipoint): number {         return distance(this, a);     }     constructor(x: number, y: number) {         this.x = x;         this.y = y;     } }   function distance(p1: ipoint, p2: ipoint): number {     function sq(x: number): number {         return x*x;     }     return math.sqrt( sq(p2.x-p1.x)+sq(p2.y-p1.y) ); }  assert(distance ( new point(0,0), new point(3,4))===5); // distance ( new point(3,3), 3); // flow complains, expected assert((new point(0,1)).distanceto(new point(3,5))===5); // (new point(0,1)).distanceto(3); // flow complains expected 

running npm run flow yields no complains expected, whereas commented-out lines give rise warnings (again, expected).

so all's world except don't know how make explicit @ point class point defined "implementing" interface ipoint. there way or not idiomatic?

here simplest way it:

class point {     x: number;     y: number;     constructor(x: number, y: number) {         (this: ipoint);         this.x = x;         this.y = y;     } } 

the key part (this: ipoint). js vm's point of view it's expression nothing, flow needs check if casting this ipoint valid, checking if class implements ipoint interface.