What is the purpose of the delete operator in Javascript?

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.