type Method = 'get' | 'post';

let app = {
    get() { },
    post() { },
    foo() { },
    bar: 123
};

let methodA = 'get';
let handlerA = app[methodA]; // typeof handlerA === () => void

let methodB: Method;
let handlerB = app[methodB]; // typeof handlerB === () => void