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JavaScript

Posted by Robert Merrill
Robert Merrill
6,215 Points

Detecting if property age exists?

Hi. I'm working on this question:

Consider a object john of class Person. Which of the following is(are) the best way(s) to detect if property age exists?

A. If(john.age){ // do something } B. if ("age" in john) { // do something }

I think that the answer is obviously option 4. But I think I'm possibly missing something or there is a twist to it that I don't see. Any reason options 1, 2 or 3 could be correct?

  1. Both A & B are correct ways to detect if the property age exits for john
  2. Both A & B are incorrect ways to detect if property age exists for john
  3. Choice A is incorrect but choice B is the correct way to detect if property age exists for john
  4. Choice B is incorrect but choice A is the correct way to detect if property age exists for john

1 Answer

Catherine Gracey
Catherine Gracey
11,521 Points
Catherine Gracey
Catherine Gracey
11,521 Points

You can play around with this in your JavaScript console. This is what my console reports:

var test = {key: "value"}
if (test.key) console.log("true") //Printed "true"
if ("key" in test) console.log("true") //Printed "true"
if (test.missingkey) console.log("true") //Printed nothing
if ("missingkey" in test) console.log("true") //Printed nothing

Since A and B both evaluate to true, answer 1 could be correct.