hasOwnProperty() example

gistfile1.js

function rightWay(obj) {
  for (var key in obj) {
    // Ignore properties that are inherited.
    if (!obj.hasOwnProperty(key)) {
      continue;
    }
    console.log(key); 
  }
}

function wrongWay(obj) {
  for (var key in obj) {
    console.log(key); 
  }
}

// You start out with code like this.
var obj = {};
obj.x = 1;
obj.y = 2;
wrongWay(obj);
// .. which logs `x` and `y`.

// Then one day you use a new library that extends
// a base prototype without making the new property non-enumerable.
Object.prototype.answer = 42;

// Suddenly your code breaks because it is including the new property
// that it didn't expect.
wrongWay(obj);
// .. now logs `x`, `y`, and `answer` (from the prototype).

// You could have protected yourself by using
// `hasOwnProperty()` to ensure you only iterated
// over the properties on the object itself.
rightWay(obj);
// .. logs `x` and `y`, ignoring `answer` because it is on the prototype.

// Also the library author could have made `answer` non-enumerable
// to help avoid this problem but using `hasOwnProperty()`
// is still best-practice unless you /want/ to include
// the prototype's properties, of course.

thanks for the simple explanation

This is awesome explanation. To the point. Thanks 👍