The hasOwnProperty() method of objects returns a boolean indicating whether the object cnotains a specific own (non-inherited) property.

Checking for Own Properties

var obj = new Object();
obj.property = 'exists';

function modifyObj() {
  obj.newProperty = obj.property;
  delete obj.property;
}

obj.hasOwnProperty('property');  // true
modifyObj();
obj.hasOwnProperty('property');  // false

Distinguishing Own and Inherited Properties

function Constructor() {
  this.identifier = 'Constructor'
  this.greet = function () {
    console.log('Hello')
  }
}

Constructor.prototype.farewell = function () {
  console.log('Goodbye')
}

let instance = new Constructor()

console.log(instance.identifier) // Constructor
console.log(instance.hasOwnProperty('identifier')) // true
console.log(instance.hasOwnProperty('toString')) // false
console.log(instance.hasOwnProperty('hasOwnProperty')) // false
console.log(instance.hasOwnProperty('greet')) // true
console.log(instance.hasOwnProperty('farewell')) // false
console.log('farewell' in instance) // true

Iterating Over a Object's Own Properties

When examining open-source code, you frequently encounter patterns like this. The for...in loop iterates through all enumerable properties of an objectt, followed by a hasOwnProperty() check to exclude inherited properties.

var buz = {
    fog: 'stack'
};

for (var key in buz) {
    if (buz.hasOwnProperty(key)) {
        alert("this is fog (" + key + ") for sure. Value: " + buz[key]);
    }
    else {
        alert(key); // toString or other inherited properties
    }
}
  

Handling hasOwnProperty as a Property Name

JavaScript does not protect the hasOwnProperty property name, so it might exist as a property in objects that could interfere with normal behavior:

var example = {
    hasOwnProperty: function() {
        return false;
    },
    content: 'Here be dragons'
};

example.hasOwnProperty('content'); // Always returns false

// To avoid issues, use the original hasOwnProperty method from the prototype
// Call another object's hasOwnProperty with proper context
({}).hasOwnProperty.call(example, 'content'); // true

// Alternatively, use the hasOwnProperty from Object's prototype
Object.prototype.hasOwnProperty.call(example, 'content'); // true