Add customModuleResolution / customModuleResolver to compilerOptions #28624

import { a } from "./a"; // resolve to a.node.ts for example

import * as Typescript from "typescript";
import { existsSync } from "fs";
import { join, dirname } from "path";

type ResolveOutput =
  | Typescript.ResolvedModuleWithFailedLookupLocations
  | undefined;

const POSTFIXS = ["node", "browser", "modern"];

const EXTENSIONS = [
  "d.ts",
  "ts",
  "tsx",
  "js",
  "jsx",
  "abc" // custom extensions work - but not to well
];

const EXTENSIONS_MAP: { [key: string]: Typescript.Extension } = {
  "d.ts": Typescript.Extension.Dts,
  ts: Typescript.Extension.Ts,
  tsx: Typescript.Extension.Tsx,
  js: Typescript.Extension.Js,
  jsx: Typescript.Extension.Jsx,
  abc: Typescript.Extension.Ts // hack abc as ts
};

const check = (
  path: string,
  postfix: string,
  extension: string
): ResolveOutput => {
  const resolvedFileName = `${path}.${postfix}.${extension}`;
  if (existsSync(resolvedFileName)) {
    return {
      resolvedModule: {
        resolvedFileName,
        isExternalLibraryImport: false,
        extension: EXTENSIONS_MAP[extension]
      }
    };
  }
  return undefined;
};

export default (
  moduleName: string,
  containingFile: string,
  compilerOptions: Typescript.CompilerOptions
): ResolveOutput => {
  if (
    moduleName.startsWith(".") &&
    containingFile.startsWith(join(compilerOptions.baseUrl, "src"))
  ) {
    const path = join(dirname(containingFile), moduleName);
    return POSTFIXS.reduce<ResolveOutput>(
      (out, postfix) =>
        out
          ? out
          : EXTENSIONS.reduce<ResolveOutput>(
              (out, extension) => (out ? out : check(path, postfix, extension)),
              undefined
            ),
      undefined
    );
  }
  return undefined;
};