JavaScript objects do not come with a built-in hasOwnProperty function

Question

JavaScript objects do not come with a built-in hasOwnProperty function

Here is the code snippet:

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

Result :

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

Note 1: The 'res' object is from Express response;

I am using Express 4.13.3 . Does anyone know what the issue might be here?

NOTE :

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

I discovered a bug where Object.create(null) does not create a JavaScript object with built-in functions.

javascript express

Answer 1

Answer №1

res.locals is specifically defined within Express as an object that lacks a prototype:

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

By using null, the object does not inherit any properties or methods, including those from Object.prototype such as hasOwnProperty.

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

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

If you want to apply the method to res.locals, you must access it via 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'));

Answer 2

res.locals is specifically defined within Express as an object that lacks a prototype:

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

By using null, the object does not inherit any properties or methods, including those from Object.prototype such as hasOwnProperty.

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

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

If you want to apply the method to res.locals, you must access it via 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'));

JavaScript objects do not come with a built-in hasOwnProperty function

NOTE :

Answer №1

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