javascript - associative array without toString, etc...

Question

I want to create an associative array:

var aa = {} //equivalent to Object(), new Object(), etc...

and I want to be sure that any key I access is going to be a number:

aa['hey'] = 4.3;
aa['btar'] = 43.1;

I know JS doesn't have typing, so I can't automatically check this, but I can ensure in my own code that I only assign strings to this aa.

Now I'm taking keys from the user. I want to display the value for that key. However, if the user gives me something like "toString", he'll get back a function, not an int! Is there any way to make sure any string he gives me is only something I define? Is the only solution something like:

delete aa['toString'];
delete aa['hasOwnProperty'];

etc...

Answer 1

One possibility would be to use hasOwnProperty to check that the key is something you explicitly added to the array. So instead of:

function findNumber(userEnteredKey) {
    return aa[userEnteredKey];
}

you'd say:

function findNumber(userEnteredKey) {
    if (aa.hasOwnProperty(userEnteredKey))
        return aa[userEnteredKey];
}

Alternately, you could use typeof to check that anything is a number before returning it. But I like the hasOwnProperty approach, because it'll keep you from returning anything that you didn't intentionally put in the array.

Answer 2

Will this work for you?

function getValue(id){
  return (!isNaN(aa[id])) ? aa[id] : undefined;
}

Update:

With the help from Moss Collum and pottedmeat I recommend this generic solution:

function getValue(hash,key) {
    return Object.prototype.hasOwnProperty.call(hash,key) ? hash[key] : undefined;
}

Update2: Had forgot the ".call". (thanks pottedmeat for pointing that out)

Update3: (About the key)

Note the following: The key will internally be converted to a string because the key is actually a name of an attribute.

var test = {
  2:"Defined as numeric", 
  "2":"Defined as string" 
}  

alert(test[2]); //Alerts "Defined as string"

If trying to use an object:

var test={}, test2={};
test[test2]="message"; //Using an object as a key.

alert(test[test2]); //Alerts "message". Looks like it works...

alert(test[  test2.toString() ]);
//If it really was an object this would not have worked,
// but it also alerts "message".

Now that you know that it is always a string, lets use it:

var test={};

var test2={
    toString:function(){return "some_unique_value";}
    //Note that the attribute name (toString) don't need quotes.
}

test[test2]="message";
alert(test[ "some_unique_value"] ); //Alerts "message".

Answer 3

Really simple answer: when you create a new key prepend it with some string constant of your own.

var a = {};
var k = 'MYAPP.COLLECTIONFOO.KEY.';

function setkey(userstring)
{
  a[k+userstring] = 42;
}

function getkey(userstring)
{
  return a[k+userstring];
}