assert.js

function assert(expectation, fn) {
	var args = Array.prototype.slice.call(arguments, 2);
	var ret = fn.apply(fn, args);
	
	if(ret === expectation) {
		return ret;
	} else {
		throw new TypeError("调用" + fn.name + 
			"函数时，返回值" + ret + "与预期的" + expectation + "不同");
	}
}


// test case
function plus(a, b) { return a + b;}

// ok
assert(10, plus, 5, 5);

// throw error
assert(11, plus, 5, 5);