Question

The following snippet of code is taken from Eloquent JavaScript.

var noCatsAtAll = {};
if ("constructor" in noCatsAtAll)
  console.log("Yes, there definitely is a cat called 'constructor'.");

I find it quite mystifying. Why does the ‘if’ return true?

Answer 1

All instances of Object have a constructor property that specifies the function that constructs the Object’s prototype.

https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Object#Properties_2

The in operator looks at all properties, including inherited ones. If you only want to see the properties on the object itself, you can use hasOwnProperty:

var a = {};
"constructor" in a; // true
a.hasOwnProperty("constructor"); // false

Note that while the in operator sees "constructor", a for (key in a) loop wouldn’t. This is because the "constructor" property is non-enumerable.