A TypeScript custom transformer that redirects function reference to the exports object.

When targeting CommonJS as the module target, the TypeScript compiler attach all exported functions to the module.exports object. However, any reference to those functions are not modified, causing unexpected results when one tries to stub these functions in unit-tests. Consider this module:

/** foobar.ts */

export function foo() {
  return "foo";
}

export function bar() {
  return foo() + "bar";
}

which transpiles to:

/** foobar.js */

function foo() {
  return "foo";
}
exports.foo = foo;
function bar() {
  return foo() + "bar";
}
exports.bar = bar;

Let's unit test this module. If foo is a function that triggers network activity or some other operation that we do not wish to exercise, it is common to stub it with a fake function.

/** test_foobar.js */

assert.strictEqual(foobar.bar(), "foobar"); // Pass

// Use SinonJS to stub out the foo function
sinon.replace(foobar, 'foo', sinon.fake.returns("bar"));

assert.strictEqual(foobar.bar(), "barbar"); // AssertionError! Actual: foobar

Sinon works by overwriting the reference to a value on an object. The second assert failed because the foo() call in bar is referencing the said function directly rather than through the module.exports object. This package provides a custom TypeScript transformer that emits the proper function call (ie exports.foo()) so that function stubbing works as expected.

First install the package

yarn add ts-use-exports
# or
npm install ts-use-exports

Depending on your toolchain, see one of the example setups below on using the custom transformer.

ts-node by default does not provide a program instance to custom transformers. We will use ttypescript (install with yarn add ttypescript) to work around this limitation.

/** tsconfig.json */
{
  "compilerOptions": {
    // ...
    "plugins": [
      { "transform": "ts-use-exports" }
    ]
  }
}

Then invoke the ts-node binary with the ttypescipt compiler

ts-node --compiler ttypescript

Alternatively, if you use ts-node programmatically,

require('ts-node').register({
  project: 'tsconfig.json',
  compiler: 'ttypescript',
});

/** webpack.config.js */
const tsUseExports = require('ts-use-exports');

module.exports = {
  // ...
  module: {
    rules: [
      {
        test: /\.ts$/,
        loader: 'ts-loader', // or 'awesome-typescript-loader'
        options: {
          getCustomTransformers: program => ({
            before: [tsUseExports(program)],
          }),
        },
      },
    ],
  },
};

• The module target must be CommonJS (either set explicity or implied by ES3/ES5 target).

• The transformer currently does not produce the correct output if you alias a function name. See issue #3.