Chaining assert statement for JavaScript

assert.js

var assert = function(stmt) {
    var methods = {
        equals: {
            error: function(val) {
                return val + ' did not equal ' + stmt + '!'
            },
            method: function(val) {
                return val == stmt;
            }
        },
        is: {
            error: function() {
                // slow! http://bonsaiden.github.com/JavaScript-Garden/#function.arguments
                var args = Array.prototype.slice.call(arguments);

                if(args.length == 1) {
                    return stmt + ' is not a ' + args[0] + '!';
                }

                return stmt + ' not in types: ' + args.join(', ');
            },
            method: function() {
                // borrowed from RightJS
                var to_s = Object.prototype.toString;

                var typeChecks = {
                    'function': function(val) {
                        return typeof(val) === 'function';
                    },
                    string: function(val) {
                        return typeof(val) === 'string';
                    },
                    number: function(val) {
                        return typeof(val) === 'number';
                    },
                    object: function(val) {
                        return to_s.call(val) === '[object Object]';
                    },
                    array: function(val) {
                        return to_s.call(val) === '[object Array]';
                    }
                };

                for(var i = 0; i < arguments.length; i++) {
                    var value = arguments[i];

                    if(value in typeChecks) {
                        var matched = typeChecks[value](stmt);

                        if(matched) {
                            return true;
                        }
                    }
                }

                return false;
            }
        },
        isDefined: {
            error: function() {
                return 'Expected a defined value, got undefined instead!';
            },
            method: function() {
                // borrowed from RightJS
                return typeof(stmt) !== 'undefined';
            }
        },
        between: {
            error: function(a, b) {
                return stmt + ' was not between ' + a + ' and ' + b + '!'
            },
            method: function(a, b) {
                return a <= stmt && stmt <= b;
            }
        },
        not: {
            error: function() {},
            method: function() {
                invertNext = !invertNext;

                return true;
            }
        }
    };
    var ret = {};
    var invertNext = false;

    var insertMethod = function(name, data) {
        ret[name] = function() {
            var success = data.method.apply(this, arguments);

            if(name != 'not' && invertNext) {
                success = !success;
                invertNext = false;
            }
            
            if(success) {
                return ret;
            }

            // http://aymanh.com/9-javascript-tips-you-may-not-know#assertion
            function AssertionError(message) {
                this.message = message;
            }
            AssertionError.prototype.toString = function () {
                return 'AssertionError: ' + this.message;
            }

            var errorText = data.error.apply(this, arguments);
            throw new AssertionError(errorText);
        }
    }

    for(var methodName in methods) {
        var methodData = methods[methodName];

        insertMethod(methodName, methodData);
    }

    return ret;
}

var a = 5;

// these should pass
assert(a).equals(5);
assert(a).not().equals(10);
assert(a).not().not().equals(5);
assert(a).between(0, 10);
assert(a).isDefined();
assert(a).is('number');
assert(a).is('array', 'number');

// these should fail
assert(a).is('array', 'object');
assert(a).equals('foobar');

assert.md

Ideas

https://gist.github.com/clupasq/6144b04373dd01d514e6

assert.min.js

var assert=function(n){var t={equals:{error:function(t){return t+" did not equal "+n+"!"},method:function(t){return t==n}},is:{error:function(){var t=Array.prototype.slice.call(arguments);return 1==t.length?n+" is not a "+t[0]+"!":n+" not in types: "+t.join(", ")},method:function(){for(var t=Object.prototype.toString,r={function:function(n){return"function"==typeof n},string:function(n){return"string"==typeof n},number:function(n){return"number"==typeof n},object:function(n){return"[object Object]"===t.call(n)},array:function(n){return"[object Array]"===t.call(n)}},e=0;e<arguments.length;e++){var o=arguments[e];if(o in r&&r[o](n))return!0}return!1}},isDefined:{error:function(){return"Expected a defined value, got undefined instead!"},method:function(){return void 0!==n}},between:{error:function(t,r){return n+" was not between "+t+" and "+r+"!"},method:function(t,r){return t<=n&&n<=r}},not:{error:function(){},method:function(){return e=!e,!0}}},r={},e=!1,o=function(n,t){r[n]=function(){var o=t.method.apply(this,arguments);if("not"!=n&&e&&(o=!o,e=!1),o)return r;function i(n){this.message=n}i.prototype.toString=function(){return"AssertionError: "+this.message};var u=t.error.apply(this,arguments);throw new i(u)}};for(var i in t){var u=t[i];o(i,u)}return r};