From [Understanding Delete][] by Juriy Zaytsev (2010)

delete is a built-in JS operator for removing properties from objects:

var foo = { bar: 'baz' }
delete foo.bar  // true
foo             // {}

There’s a lot of misinformation floating around on the Internet about what it is and how it works. In Object-Oriented JavaScript, Stoyan Stefanov even mistakenly writes that it can be used to delete bare functions:

function foo() { console.log('bar') }  // AUTHOR SAYS | ACTUALLY RETURNS
delete foo                             // true        | false
typeof foo                             // undefined   | function

So what’s the confusion all about?

Background

In JS, all code is run within an execution context. This determines what variables are accessible at any given time. Usually, when you declare and use variables or functions, they appear to exist on their own:

var foo = 1
foo  // 1

function bar() { return 1 }
bar()  // 1

but in reality, they are defined as properties on the global object (in global code) or activation object (in function code):

// NOTE: this does not apply for const and let
var foo = 1
typeof this.foo  // 'number'

function bar() { return 1 }
typeof this.bar  // 'function'

Within functions, this object is also where function parameters (and the arguments variable) are stored.

For a comprehensive take, see this lesson from Tyler McGinnis’ Advanced JavaScript course. (Side note: can’t recommend his courses enough. Worth every penny.)

How delete Really Works

delete doesn’t operate on variables; it operates on object properties. Under the hood, object properties have certain flags set on them at the time they are defined; one such property is DontDelete. You can probably guess what it does.

DontDelete is only disabled when 1) a property is explicitly assigned or 2) a variable is set via undeclared assignment:

var foo = 1           // DontDelete
function foo()        // DontDelete
this.foo = 1          // !DontDelete
foo = 1               // !DontDelete
var foo.bar = 1       // foo: DontDelete / bar: !DontDelete
var foo = { bar: 1 }  // foo: DontDelete / bar: !DontDelete

In practice, browsers have all sorts of quirks when implementing this behavior.

Why Object-Oriented JavaScript Was Mistaken

Presumably, the author was using Firebug to confirm his findings. Firebug runs all code in an eval execution context, where even more inconsistencies abound!