Contracts implemented in ~25 LOC

contracts.js.es6

/*
 * Released under the MIT License
 *
 * Copyright (c) 2014 Peter Lejeck
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to
 * deal in the Software without restriction, including without limitation the
 * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
 * sell copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 */

/**
 * Tests an assertion
 * @private
 */
function _assert (assertion, args, msg, src, cond) {
	let result = assertion(...args);
	let message = cond + 'Condition failed for ';
	message += src ? 'function "' + src + '"' : 'Anonymous Function';
	message += ': ' + msg;
	throw new Error(message);
}

/**
 * Generates a fallback message using the function return statement
 * @private
 */
function _functionMessage (fn) {
	let str = fn.toString();
	let returnStart = str.indexOf('return') + 'return '.length;
	let returnEnd = str.indexOf(';', returnStart) || str.indexOf('}', returnStart);

	return str.substring(returnStart, returnEnd).trim();
}

/**
 * Ensure a function's return value is valid
 *
 * If the function ever returns a value which the fn returns false for,
 * execution is interrupted (by an error) and the msg is displayed in the
 * console.  If there is no msg provided, one will be generated from the return
 * statement in the assertion body.
 */
Function.prototype.ensures = function ensures (fn, msg = null) {
	if (msg == null) msg = _functionMessage(fn);
	return () => {
		let ret = this(...arguments);
		_assert(fn, [ret], msg, this.name, 'Post');
		return ret;
	};
};

/**
 * Requires that a parameter is valid
 *
 * If the function ever recieves a value which the fn returns false for,
 * execution is interrupted (by an error) and the msg is displayed in the
 * console.  If there is no msg provided, one will be generated from the return
 * statement in the assertion body.
 */
function requires (fn, msg = null) {
	if (msg == null) msg = functionMessage(fn);
	_assert(fn, [], msg, arguments.caller.name, 'Pre');
}

example.js

import requires from 'contracts-development.js';

// Will error with "PostconditionFailed: r != false"
var failure = function failure () {
  return false;
}.ensures((r)=> r != false)
failure();

// And have some preconditions
var failure = function failure (a, b) {
  requires(()=> a > b);
  return a - b;
};
failure(1, 5);

prod-contracts.js.es6

Function.prototype.ensures = function ensures () { return this; };
export default function requires () {};