Demo Code from What's New in TypeScript (BRK2150) at Microsoft Build 2018

dynamicService.ts

export type TypeFromConstructor<T> =
    T extends StringConstructor ? string :
    T extends NumberConstructor ? number :
    any;
export type MethodDefinition = {
    [x: string]: StringConstructor | NumberConstructor;
};
export type ServiceDefinition = {
    [x: string]: MethodDefinition
};
export type ServiceMethod<T extends MethodDefinition> =
    {} extends T ? () => boolean : (payload: RequestPayload<T>) => boolean;
export type ServiceObject<T extends ServiceDefinition> = {
    [P in keyof T]: ServiceMethod<T[P]>
};
export type RequestPayload<T extends MethodDefinition> =
    {} extends T ? undefined : { [P in keyof T]: TypeFromConstructor<T[P]> };
export type RequestHandler<T extends ServiceDefinition> =
    (req: RequestObject<T>) => boolean;
export type RequestObject<T extends ServiceDefinition> = {
    [P in keyof T]: { message: P, payload: RequestPayload<T[P]> }
}[keyof T];

export function createService<S extends ServiceDefinition>(serviceDef: S, handler: RequestHandler<S>): ServiceObject<S> {
    const service = {} as any;
    for (const name in serviceDef) {
        service[name] = (payload: any) => handler({ message: name, payload });
    }

    return service as ServiceObject<S>;
}

usage.ts

import { createService } from './dynamicService';

const serviceDefinition = {
    open: { filename: String },
    insert: { pos: Number, text: String },
    delete: { pos: Number, len: Number },
    close: {}
};
const service = createService(serviceDefinition, req => {
    switch (req.message) {
        case 'open':
            console.log('filename: ' + req.payload.filename);
            break;
        case 'insert':
            console.log('pos: ' + req.payload.pos);
            console.log('text: ' + req.payload.text);
            break;
        case 'delete':
            console.log('pos: ' + req.payload.pos);
            console.log('len: ' + req.payload.len);
            break;
        case 'close':
            // This should be undefined.
            console.log('payload: ' + req.payload);
            break;
    }

    return true;
});

service.open({ filename: 'test.txt' });
service.insert({ pos: 0, text: 'hello world' });
service.delete({ pos: 0, len: 5 });
service.close();