CODEWITHSUNDEEP
javascript
deltanovember
deltanovember

Reputation: 44091

In JavaScript, where does "constructor" come from?

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?

Upvotes: 2

Views: 89

Answers (4)

uadnal
uadnal

Reputation: 11445

constructor is a method of Object. You can find the constructor method throughout all objects unless you modify it. The in operator will find methods through the prototype chains. Which is is why it's recommended to use hasOwnProperty to test for properties in your own objects. 

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

    if ('constructor' in Object)
       console.log("Yes, there is also a method called constructor");

    var noCon = Object.create(null); // create a completetly empty object
    console.log('constructor' in noCon); // false

    function hasConstructorToo() {}

    console.log('constructor' in hasConstructorToo) // true
    console.log('constructor' in []); // true

http://jsfiddle.net/Xsb3E/3`

Upvotes: 1

Tikhon Jelvis
Tikhon Jelvis

Reputation: 68172

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.

Upvotes: 5

kprobst
kprobst

Reputation: 16661

JavaScript objects have a function called constructor which is the function that created the object's instance. It's built-in to all objects. The in operator tests for the presence of something called "constructor" in the instance of your dictionary, so it returns true. The same thing would happen if you tested for length, for example.

Upvotes: 6

Matthew Flaschen
Matthew Flaschen

Reputation: 285077

It is the constructor of the Object type. A reference to the constructor function is available directly on that property ("constructor") of an object (it applies to constructors you write too).

In turn, the names of properties present are in objects.

Upvotes: 2

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