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History
This warning has existed in two forms in JSLint, JSHint and ESLint. It was
introduced in the original version of JSLint and has remained in all three tools
ever since.

In JSLint the warning given is "Only properties should be deleted"

In JSHint and ESLint the warning has always been "Variables should not be
deleted"


The situations that produce the warning have not changed despite changes to the
text of the warning itself.
When do I get this error?
The "Only properties should be deleted" error, and the alternative "Variables
should not be    deleted" error, is thrown when JSLint, JSHint or ESLint
encounters the delete operator followed by a single identifier. In the
following example we declare a variable x and then attempt to delete it:
​x
 
1
var x = 10;
2
delete x;
3
​
JSLint found 1 error
Which linter should be used for this code snippet:
 JSLint JSHint ESLint
Which version of the linter should be used:
Update
Version 2015-09-23
Line 1:
Expected '.' and instead saw 'x'.

Why do I get this error?
This error is raised to highlight code that probably doesn't work as you
expect it to. It can also indicate a fatal syntax error. The delete
operator will only delete properties of objects. It cannot "delete" variables or
anything else. Here's a valid use of the delete operator. Notice how this time
there are no JSLint errors:
5
 
1
var x = {
2
    prop: 10
3
};
4
delete x.prop;
5
​
JSHint found no errors
Which linter should be used for this code snippet:
 JSLint JSHint ESLint
Which version of the linter should be used:
Update
Version 2.9.0

The ECMAScript 5 specification details the behaviour of the delete operator
(ES5 §11.4.1). When the operand is a reference to an object
property this is what happens:

If IsPropertyReference(ref) is true, then

    Return the result of calling the [[Delete]] internal
method on ToObject(GetBase(ref)) providing GetReferencedName(ref) and
IsStrictReference(ref) as the arguments.

But when the operand is a reference to an Environment Record binding (something
that is not an object property), the runtime will attempt to delete it (and
fail) unless the code is running in strict mode. In that case a syntax error is
thrown:

Else, ref is a Reference to an Environment Record binding, so

    If IsStrictReference(ref) is true, throw a SyntaxError
exception.

    ...[attempt to delete]...

In JSHint 1.0.0 and above you have the ability to ignore any warning with a
special option syntax. The identifier of this warning is W051.
This means you can tell JSHint to not issue this warning with the /*jshint
-W051 */ directive.