SyntaxError: applying the 'delete' operator to an unqualified name is deprecated

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: Calling delete on expression not allowed in strict mode (Edge)
SyntaxError: applying the 'delete' operator to an unqualified name is deprecated (Firefox)
SyntaxError: Delete of an unqualified identifier in strict mode. (Chrome)

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, doesn't work in JavaScript and it 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