JavaScript: The Definitive Guide, 6th Edition

Deleting Properties

The delete operator (The delete Operator) removes a property from an object. Its single operand should be a property access expression. Surprisingly, delete does not operate on the value of the property but on the property itself:

delete book.author;          // The book object now has no author property.
delete book["main title"];   // Now it doesn't have "main title", either.

The delete operator only deletes own properties, not inherited ones. (To delete an inherited property, you must delete it from the prototype object in which it is defined. Doing this affects every object that inherits from that prototype.)

A delete expression evaluates to true if the delete succeeded or if the delete had no effect (such as deleting a nonexistent property). delete also evaluates to true when used (meaninglessly) with an expression that is not a property access expression:

o = {x:1};          // o has own property x and inherits property toString
delete o.x;         // Delete x, and return true
delete o.x;         // Do nothing (x doesn't exist), and return true
delete o.toString;  // Do nothing (toString isn't an own property), return true
delete 1;           // Nonsense, but evaluates to true

delete does not remove properties that have a configurable attribute of false. (Though it will remove configurable properties of nonextensible objects.) Certain properties of built-in objects are nonconfigurable, as are properties of the global object created by variable declaration and function declaration. In strict mode, attempting ...