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)

SyntaxError instrict mode only.

Normal variables in JavaScript can't be deleted using thedelete operator. In strict mode, an attempt to delete a variable will throw an error and is not allowed.

Thedelete operator can only delete properties on an object. Object properties are "qualified" if they are configurable.

Unlike what common belief suggests, thedelete operator hasnothing to do with directly freeing memory. Memory management is done indirectly via breaking references, see thememory management page and thedelete operator page for more details.

This error only happens instrict mode code. In non-strict code, the operation just returnsfalse.

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 tonull:

"use strict";var x;// …x = null;// x can be garbage collected

• delete
• Memory management
• TypeError: property "x" is non-configurable and can't be deleted