Package Exports

babel-plugin-typecheck

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Readme

Babel Typecheck

This is a Babel plugin for static and runtime type checking using flow type annotations.

What?

Turns code like this:

function sendMessage (to: User, message: string): boolean {
  return socket.send(to, message);
}

into code like this:

function sendMessage(to, message) {
  var _socket$send;

  if (!(to instanceof User)) throw new TypeError("Value of argument 'to' violates contract.");
  if (typeof message !== "string") throw new TypeError("Value of argument 'message' violates contract.");
  _socket$send = socket.send(to, message);
  if (typeof _socket$send !== "boolean") throw new TypeError("Function 'sendMessage' return value violates contract.");
  return _socket$send;
}

And guards against some silly mistakes, for example compiling the following code will raise a SyntaxError, because the function can return the wrong type.

function foo (): boolean {
  if (Math.random() > 0.5) {
    return "yes"; // <-- SyntaxError - string is not boolean
  }
  else {
    return false;
  }
}

function bar (input: string = 123): string { // <-- SyntaxError: default value is not string
  return input + "456";
}

In cases where typecheck can statically verify that the return value is of the correct type, no type checks will be inserted, for instance:

function bar (): string|Object {
  if (Math.random() > 0.5) {
    return "yes";
  }
  else {
    return {
      message: "no"
    };
  }
}

will produce no type checks at all, because we can trivially tell that the function can only return one of the two permitted types. This is also true for simple cases like:

function createUser (): User {
  return new User(); // <-- no typecheck required
}

This is currently quite limited though, as the plugin can only statically infer the types of literals and very simple expressions, it can't (yet) statically verify e.g. the result of a function call. In those cases a runtime type check is required:

function createUser (): User {
  return User.create(); // <-- produces runtime typecheck
}

Installation

First, install via npm.

npm install --save-dev babel-plugin-typecheck

Then, in your babel configuration (usually in your .babelrc file), add "typecheck" to your list of plugins:

{
  "plugins": ["typecheck"]
}

License

Published by codemix under a permissive MIT License, see LICENSE.md.