Object javascript dont have build-in function hasOwnProperty

Question

Here my code

console.log(typeof res.locals);
console.log(res.locals.hasOwnProperty('a'));

My result :

object
Unhandled rejection TypeError: res.locals.hasOwnProperty is not a function

Note 1: res is response Object by Express;

I use Express 4.13.3 .Anyone know what problem here ?

NOTE :

  var a = Object.create(null);
  var b = {};
  a.hasOwnProperty('test');
  b.hasOwnProperty('test');

I find bug here Object.create(null) dont make Object javascript with buildin function

Answer 1

res.locals is defined in Express as an object without a prototype:

res.locals = res.locals || Object.create(null);

By passing null , the object doesn't inherit any properties or methods, including those on Object.prototype like hasOwnProperty .

console.log(Object.getPrototypeOf(res.locals)); // null

console.log(Object.create(null) instanceof Object); // false

---

To use the method with res.locals , you'll have to access it through the Object.prototype :

console.log(Object.prototype.hasOwnProperty.call(res.locals, 'a'));

// or store it
var hasOwnProperty = Object.prototype.hasOwnProperty;
console.log(hasOwnProperty.call(res.locals, 'a'));