Decorators for Javascript a la Python

README.md

Decorators for Javascript ala Python

Imagine a function named foo that takes two parameters, prints them to a console, and returns a boolean true if the two parameters are equal:

function foo(a, b) {
    console.log([a, b]);
    return a === b;
}

A test run produces the following output:

> foo(1, 2);
[1, 2]
false

The decorator accepts enforces the type of the arguments passed in. It can be used to require the two arguments to strings and return a boolean.

decorate(window, "foo", accepts("string", "string"), returns("boolean"));

Now the previous test run will produce an error:

> foo(1, 2);
TypeError: arg 0 does not match string

References

• http://www.python.org/dev/peps/pep-0318/#examples

decorator-demo.js

function decorate() {
    var args = Array.prototype.slice.call(arguments);
    var obj = args.shift(),
        fnName = args.shift();
    if (!(obj && fnName)) {
        return;
    }
    var fn = obj[fnName];
    for (var i = 0; i < args.length; i++) {
        var wrap = args[i];
        fn = wrap(fn);
    }
    obj[fnName] = fn;
}

function accepts() {
    var types = Array.prototype.slice.call(arguments);
    return function(fn) {
        var newFn = function() {
            var args = Array.prototype.slice.call(arguments);
            if (args.length != types.length) {
                throw "not enough arguments";
            }
            for (var i = 0; i < args.length; i++) {
                var arg = args[i],
                    type = types[i];
                if (typeof arg != type) {
                    throw new TypeError("arg " + i +
                                        " does not match " + type);
                }
            }
            return fn.apply(fn, args);
        };
        return newFn;
    };
}

function returns(rtype) {
    return function(f) {
        function newFn() {
            var args = Array.prototype.slice.call(arguments);
            var result = f.apply(fn, args);
            if (typeof result != rtype) {
                throw new TypeError("return value " + result +
                                    " does not match" + rtype);
            }
            return result;
        }
        return newFn;
    };
}