Babel label decorator

This is a Babel plugin for design by contract for JavaScript.

What?

Installation

Install via npm.

npm install --save-dev babel-plugin-label-decorator

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

{
  "plugins": [
    ["label-decorator", {
      "env": {
        "production": {
          "strip": true
        }
      }
    }]
  ]
}

The above example configuration will remove all label-decorator when NODE_ENV=production, which is often preferable for performance reasons. You can customize the names of the labels and identifiers by specifying a names option, e.g.

{
  "plugins": [
    ["label-decorator", {
      "names": {
        "assert": "assert",
        "precondition": "pre",
        "postcondition": "post",
        "invariant": "invariant",
        "return": "it",
        "old": "old"
      }
    }]
  ]
}

Examples

1. Precondition Only.

The contract for the following function specifies that the first argument must always be a string.

function warn (message) {
  pre: typeof message === 'string';
  return 'Warning!\n' + message;
}

If we call this function with a non string argument, an error will be thrown.

2. Postcondition Only.

The following function specifies that the result of the function must always be an array containing more than one element.

Note: Post-conditions introduce a special variable, it which refers to the result of the function.

function items (a, b) {
  let c = [];
  if (a) {
    c.push(a);
  }
  if (b) {
    c.push(b);
  }
  return c;

  post: {
    Array.isArray(it);
    it.length > 0;
  }
}

If we call this function without arguments, the post-condition will fail and an error will be thrown.

Note: preconditions and postconditions can appear in any order directly within the function body.

Postconditions can also refer to the state of the world at the entry point of the function, which is extremely useful when verifying the results of functions with side effects. For this, we use a pseudo-function called old() which takes a single argument - the reference we want to capture, for example:

function applyDiscount (cart, amount) {
  pre: {
    !cart.hasDiscount, "Discounts can only be applied once";
    cart.total >= amount, "Cannot discount to less than zero.";
  }
  post: {
    cart.total === old(cart.total) - amount;
  }
  cart.total -= amount;
  cart.hasDiscount = true;
  // some more complicated stuff goes here...
  return cart;
}

3. Preconditions and Postconditions.

function withdraw (fromAccount, amount) {
  pre: {
    typeof amount === 'number';
    amount > 0;
    fromAccount.balance - amount > -fromAccount.overdraftLimit;
  }
  post: {
    fromAccount.balance - amount > -fromAccount.overdraftLimit;
  }

  fromAccount.balance -= amount;
}

4. Invariants

Invariants run at the beginning and end of a block. Using invariants we can simplify the above example.

function withdraw (fromAccount, amount) {
  pre: {
    typeof amount === 'number';
    amount > 0;
  }
  invariant: {
    fromAccount.balance - amount > -fromAccount.overdraftLimit;
  }

  fromAccount.balance -= amount;
}

4. Assertions

Assertions verify that something is truthy and throw an error if the assertion fails. They run where they are specified:

function add (a, b) {
  const result = a + b;
  assert: typeof result === 'number';
  return result;
}

or, with multiple:

function addAndSquare (a, b) {
  let result = a + b;
  assert: {
    typeof result === 'number';
    !isNaN(result);
  }

  result *= result;

  assert: result < Math.pow(2, 32), "Must be within an acceptable range";

  return result;
}

5. Error Messages

Often it's nice to provide an error message for the contract that failed, for example:

function withdraw (fromAccount, amount) {
  pre: {
    typeof amount === 'number', "Second argument must be a number";
    amount > 0, "Cannot withdraw a zero or negative amount";
    fromAccount.balance - amount > -fromAccount.overdraftLimit, "Must not exceed overdraft limit";
  }
  post: {
    fromAccount.balance - amount > -fromAccount.overdraftLimit, "Must not exceed overdraft limit";
  }

  fromAccount.balance -= amount;
}

Now if a contract fails, the error object will have a descriptive message.

License

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