Literal Union Types => Typeof Unions in Property Indexes

[ethanresnick]

TypeScript Version: 2.1.0-dev.dev.20160920

Code

import express = require("express");
const app = express();

const method = (someCondition()) ? "get" : "post"; // method is "get" or "post"
app[method](reqHandler);

Expected behavior:
Ideally, app[method] could be inferred as typeof app.get | typeof app.post

Actual behavior:
app[method] gives an implicit any error, since app doesn't have an index signature. Doing (<any>app)[method] removes the error, but leaves app[method] as type any rather than typeof app.post | typeof app.get.

To get proper typing, I can do: (<typeof app.get | typeof app.post>(<any>app)[method]), but that's super ugly.

The below almost works:

const method = (someCondition()) ? app.get.bind(app) : app.post.bind(app);
method(reqHandler);

except that .bind() calls return any.

[RyanCavanaugh]

Duplicate of #6080 (it even uses get / post 😄 )

[ethanresnick]

Lol. Closing in favor of that one.