The Difference Between in and hasOwnProperty in JavaScript

Given a general JavaScript object, there are two common ways to check whether an object contains a key: the in operator and the hasOwnProperty() function. With a simple POJO and no special keys, these two are equivalent:

const obj = { answer: 42 };
'answer' in obj; // true
obj.hasOwnProperty('answer'); // true

'does not exist' in obj; // false
obj.hasOwnProperty('does not exist'); // false

Both also support ES6 symbols.

const symbol = Symbol('answer');
const obj = { [symbol]: 42 };

symbol in obj; // true
obj.hasOwnProperty(symbol); // true

So what's the difference between the two? The key difference is that in will return true for inherited properties, whereas hasOwnProperty() will return false for inherited properties.

For example, the Object base class in JavaScript has a __proto__ property, a constructor property, and a hasOwnProperty function. The in operator will return true for these properties, but hasOwnProperty() will return false.

'constructor' in obj; // true
'__proto__' in obj; // true
'hasOwnProperty' in obj; // true

obj.hasOwnProperty('constructor'); // false
obj.hasOwnProperty('__proto__'); // false
obj.hasOwnProperty('hasOwnProperty'); // false

Because hasOwnProperty() ignores inherited properties, it is the better choice for plain old JavaScript objects (POJOs). However, hasOwnProperty() will return false for ES6 class getters and methods, like ES6 getters.

class BaseClass {
  get baseProp() {
    return 42;
  }
}
class ChildClass extends BaseClass {
  get childProp() {
    return 42;
  }
}
const base = new BaseClass();
const child = new ChildClass();

'baseProp' in base; // true
'childProp' in child; // true
'baseProp' in child; // true

base.hasOwnProperty('baseProp'); // false
child.hasOwnProperty('childProp'); // false
child.hasOwnProperty('baseProp'); // false

Below is a summary of the tradeoffs between in and hasOwnProperty().

In general, hasOwnProperty() is the right choice most of the time, because you avoid issues with special keys, like constructor. A good rule of thumb is that if you're looking to see whether an object has a property, you should use hasOwnProperty(). If you're looking to see if an object has a function that you intend to call, like checking if an object has toString(), you should use in.


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