add.js

function add(x, y) {
    return x + y;
}
exports.add = add;

add.ts

export function add(x:number, y:number): number {
    return x + y;
}

package.json

{
  "name": "tmp3",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "build": "tsc -t es5 -m commonjs add.ts",
    "test": "mocha --reporter=dot test.js"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "mocha": "^2.3.4",
    "typescript": "^1.7.5"
  }
}

test.js

var assert = require('assert');

it("add 3 + 3 = 6", function(){
  var add = require('./add').add;
  assert.strictEqual(add(3, 3), 6);
});