Question

I need to check whether a class definition provides either via inheritance or not, a specific method. Do I need to travel the prototype chain to accomplish this?

function TestClass(config){
    //NOTE: cannot instantiate class because if config not valid Error is thrown
}
TestClass.prototype.sampleMethod = function(){};

function isDefined(klass){
      console.log(typeof klass.sampleMethod); //'undefined'
      console.log('sampleMethod' in klass); //false
      console.log(klass['sampleMethod']); //undefined

      console.log(typeof klass.prototype.sampleMethod); //'function' ...inheritance?
}

isDefined(TestClass);

Answer 1

Yes, you need to check the prototype chain.

function TestClass(config){}
TestClass.prototype.sampleMethod = function(){};

function TestClass2() {}
TestClass2.prototype = TestClass;

function isDefined(obj, method) {
    if (obj.prototype !== undefined) {
        var methodInPrototype = (method in obj.prototype);
        console.log("Is " + method + " in prototype of " + obj + ": " + methodInPrototype);
        if (methodInPrototype) {
                return true;
        } else {
            isDefined(obj.prototype, method);
        }
    }
    return false;
}

isDefined(TestClass, "sampleMethod");
isDefined(TestClass2, "sampleMethod");
isDefined(TestClass2, "sampleMethod2");

Prints:

// Is sampleMethod in prototype of function TestClass(config) {}: true
// Is sampleMethod in prototype of function TestClass2() {}: false
// Is sampleMethod in prototype of function TestClass(config) {}: true
// Is sampleMethod2 in prototype of function TestClass2() {}: false
// Is sampleMethod2 in prototype of function TestClass(config) {}: false