using delete

[Andrew Poulos]

Why is it that if I do this:

Foo = function() {
this.prop = true;
};
Foo.prototype.hi = function() {
alert("hi");
};
bar = new Foo();
delete bar;
bar.hi();

nothing gets alerted. But if I change the call to the constructor to

var bar = new Foo();

then the alert gets fired. And if I do this:

Foo = function() {
this.prop = true;
};
Foo.prototype.hi = function() {
delete this;
};
bar = new Foo();
bar.hi();

the object 'bar' does not seem to have been deleted? I'm testing with FF 2.

Andrew Poulos

 

[Richard Cornford]

Why is it that if I do this:

Foo = function() {
this.prop = true;};

Foo.prototype.hi = function() {
alert("hi");};

bar = new Foo();
delete bar;
bar.hi();

nothing gets alerted.

But you do get an error report (you should look at/for error reports).

But if I change the call to the constructor to

var bar = new Foo();

then the alert gets fired. And if I do this:

Variables declared in function or global execution contexts are marked
with the - DontDelete - attribute and so cannot be deleted (the delete
operation evaluates as false instead of true to indicate its falure
(sort of)).

Foo = function() {
this.prop = true;};

Foo.prototype.hi = function() {
delete this;};

bar = new Foo();
bar.hi();

Why would it? You have attempted to delete - this -, which is not
possible as - this - does not evaluate as a Reference type (it is a
value that is a reference to an object), and so the delete operator
does not act at all (but the delete expression still evaluates as true
in this case).

In any event, the - delete - operator works only by removing
properties from objects (where they are not marked as - DontDelete -)
and whatever object is referred to by - this - does not know which
properties of other objects (including Variable objects) may refer to
it, if any).

Richard.

 

[pcx99]

Variables declared in function or global execution contexts are marked
with the - DontDelete - attribute and so cannot be deleted (the delete
operation evaluates as false instead of true to indicate its falure
(sort of)).

Interesting gotcha.

 

[Richard Cornford]

Interesting gotcha.

Why is that a "gotcha"? There is no particularly good reason for wanting
to delete declared variables so not being able to is is hardly an
inconvenience.

Richard.