What is the purpose of the delete operator in Javascript?

Question

The behaviour of the delete operator seems very complicated and there are many misunderstandings about what it actually does. To me, it seems that reassigning something to undefined will more reliably do what you would expect.

I've never seen the delete keyword in Javascript actually used in non-example code and I am wondering if it is particularly useful for anything. Does delete have any purpose that cannot be acheived by reassignment to undefined? Is it used at all in any of the famous libraries (e.g. jQuery, dojo, backbone, etc)?

Answer 1

Does delete have any purpose that cannot be acheived by reassignment to undefined?

Yes. If you want to unmask a property from a prototype or cause in, hasOwnProperty, and for (...in...) to not record the property as existing then delete is appropriate.

let set = {};

set._x = true;

alert('_x' in set);  // true

set._x = undefined;

alert('_x' in set);  // true

delete set._x;

alert('_x' in set);  // false

EDIT: As T.J. Crowder explains:

The purpose of the delete operator is to completely remove a property from an object, whereas setting a property to undefined just sets the property to undefined.

This matters in its own right, but it also matters when you're using inheritance, because if O derives from P

let P = { prop: 42 };
let O = Object.create(P);  // P is O's prototype.

when you retrieve O.prop, you get the value of prop from O if O has a property with that name (even if its value is undefined), but if O doesn't have the property at all, then the value will be retrieved from P.prop instead.

console.log(O.prop);  // "42" since O doesn't have its own prop, but P does.
O.prop = undefined;
console.log(O.prop);  // "undefined" since O has its own prop.
delete O.prop;
console.log(O.prop);  // "42" since the delete "unmasked" P.prop.