Exploration of object type detection by equality testing of member methods (notably toString).

gistfile1.js

function isArray(obj) {
	return obj && obj.toString == Array.prototype.toString
}
function isNumber(obj) {
	return obj && obj.toString == Number.prototype.toString
}
function isString(obj) {
	return obj && obj.toString == String.prototype.toString
}
function isArguments(obj) {
	return obj && Object.prototype.toString.call(obj) == '[object Arguments]'
}
function isObject(obj) {
	return obj && obj.toString == Object.prototype.toString && !isArguments(obj)
}


var identifyingFunctions = [isObject, isArray, isNumber, isString]

test({}, isObject)
test([], isArray)
test(1, isNumber)
test('a', isString)
var arguments = (function() { return arguments }())
test(arguments, isArguments)

function test(obj, correctIdentifyingFunction) {
	for (var i=0, identifyingFunction; identifyingFunction = identifyingFunctions[i]; i++) {
		if (identifyingFunction == correctIdentifyingFunction) {
			assert(identifyingFunction(obj))
		} else {
			assert(!identifyingFunction(obj))
		}
	}
}

function assert(truthy) {
	if (!truthy) { throw new Error("Assertion failed") }
}