The JavaScript strict mode-only exception "applying the 'delete' operator to an unqualified name is deprecated" occurs when variables are attempted to be deleted using the delete operator.
Message
SyntaxError: Delete of an unqualified identifier in strict mode. (V8-based)
SyntaxError: applying the 'delete' operator to an unqualified name is deprecated (Firefox)
SyntaxError: Cannot delete unqualified property 'a' in strict mode. (Safari)
Error type
SyntaxError in strict mode only.
What went wrong?
Normal variables in JavaScript can't be deleted using the delete operator. In strict mode, an attempt to delete a variable will throw an error and is not allowed.
The delete operator can only delete properties on an object. Object properties are "qualified" if they are configurable.
Unlike what common belief suggests, the delete operator has nothing to do with directly freeing memory. Memory management is done indirectly via breaking references, see the memory management page and the delete operator page for more details.
This error only happens in strict mode code. In non-strict code, the operation just returns false.
Examples
Freeing the contents of a variable
Attempting to delete a plain variable throws an error in strict mode:
"use strict";
var x;
// …
delete x;
// SyntaxError: applying the 'delete' operator to an unqualified name
// is deprecated
To free the contents of a variable, you can set it to null:
"use strict";
var x;
// …
x = null;
// x can be garbage collected