Question

I usually test to see if a string is a key in an object by asking:

var foo:Object = {bar:"bar", bah:["bah","bah1"]};
var str:String = "boo";
if(foo[str]) // do something

But if str == “constructor” it will do something – because foo[“constructor”] returns true no matter what.

What is the best way to test if a string is the key of an object – and does not return true for constructor?

Some examples:

var foo:Object = {bar:"bar", bah:["bah","bah1"]};
trace('foo["bar"]: ' + foo["bar"]);
trace('foo["bah"]: ' + foo["bah"]);
trace('foo["constructor"]: ' + foo["constructor"]);
trace('foo["bar"] == true: ' + (foo["bar"] == true));
trace('foo["bah"] == true: ' + (foo["bah"] == true));
trace('foo["constructor"] == true: ' + (foo["constructor"] == true));
if(foo["bar"]){
    trace("foo:bar");
}
if(foo["bah"]){
    trace("foo:bah");
}
if(foo["constructor"]){
    trace("foo:constructor");
}
trace('"constructor" in foo: ' +  ("constructor" in foo));

Traces:

/*
foo["bar"]: bar
foo["bah"]: bah,bah1
foo["constructor"]: [class Object]
foo["bar"] == true: false
foo["bah"] == true: false
foo["constructor"] == true: false
foo:bar
foo:bah
foo:constructor
"constructor" in foo: true
*/

Answer 1

If it’s a concrete class implementation, you should be able to get away with hasOwnProperty for the most part:

var hasProperty : Boolean = foo.hasOwnProperty("bar");

Which apparently should also work fine on an anonymous object (thanks for letting me know!).