Typescript weirdness

index.ts

var a = require('./module1');
var b = require('./module2');

// import a from './module1';
// import b from './module2';

console.log('a',a);
console.log('b',b);

module1.ts

let foo = 'hello1';

module.exports = foo;

module2.ts

let foo = 'hello2';

module.exports = foo;

package.json

{
  "name": "tstest",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "",
  "license": "ISC",
  "dependencies": {
    "@types/node": "^7.0.0",
    "typescript": "^2.1.5"
  }
}

tsconfig.json

{
  "compilerOptions": {
    "outDir": "build"
  },
  "exclude": [
    "node_modules",
    "build"
  ]
}

Output:

$ bin/tsc
module1.ts(1,5): error TS2451: Cannot redeclare block-scoped variable 'foo'.
module2.ts(1,5): error TS2451: Cannot redeclare block-scoped variable 'foo'.
$ node build/index.js 
a hello1
b hello2

So the compiler throws an error but emits working code anyway.

If you comment out one of the let foo = .. lines, typescript's warnings go away, but the code it emits doesn't run. (I've commented line 1 of module2.ts below:

$ bin/tsc
$ node build/index.js 
/Users/rjmunro/personal_projects/tstest/build/module2.js:2
module.exports = foo;
                 ^

ReferenceError: foo is not defined
    at Object.<anonymous> (/Users/rjmunro/personal_projects/tstest/build/module2.js:2:18)
    at Module._compile (module.js:571:32)
    at Object.Module._extensions..js (module.js:580:10)
    at Module.load (module.js:488:32)
    at tryModuleLoad (module.js:447:12)
    at Function.Module._load (module.js:439:3)
    at Module.require (module.js:498:17)
    at require (internal/module.js:20:19)
    at Object.<anonymous> (/Users/rjmunro/personal_projects/tstest/build/index.js:2:9)
    at Module._compile (module.js:571:32)