How to see if a variable name has been defined in Javascript

Question

In Javascript, I define global variables sld_label where label can be any string. These are initialised to objects that have a setval function. I need to check, within a function which is handed label in a string parameter called qq, whether sld_label has been defined. How do I do this without raising an error?

At the moment I have the following (which works but generates an error in Firebug) :-

function setclrques(qq,op)
{
  if (typeof(eval('sld_'+qq))=='object') eval('sld_'+qq+'.setval('+op+')');
}

Help!

This has been asked many times in [SO](http://stackoverflow.com/questions/519145/how-can-i-check-whether-a-variable-is-defined-in-javascript). — The Alpha, Aug 29 '12 at 10:39

score 1 · Answer 1 · answered Aug 29 '12 at 10:38

1

If global then it ends up as member of window, so that the following works:

if (typeof window['sld_' + qq] == 'undefined') alert('sld_' + qq + ' is not defined.');

answered Aug 29 '12 at 10:38

Lloyd

29,197
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1

if the variable has been defined as false or 0 (zero), the check will fail! – daveoncode Aug 29 '12 at 10:39
Ahh yes that is true, I'll fix it. – Lloyd Aug 29 '12 at 10:39
The key to it all was using window[...] for variable variable names! Thanks! – Graphic Equaliser Aug 29 '12 at 11:36

score 0 · Answer 2 · answered Aug 29 '12 at 10:38

0

if (typeof window["sld_" + qq] === "object") {
    window["sld_" + qq].setval(op);
}

answered Aug 29 '12 at 10:38

Andreas

21,535
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This is the best solution because there is no call to "eval" - thanks! – Graphic Equaliser Aug 29 '12 at 11:16

score 0 · Answer 3 · answered Aug 29 '12 at 10:39

0

If you want to check if a variable exist

if (typeof yourvar != 'undefined') // Any scope
if (window['varname'] != undefined) // Global scope
if (window['varname'] != void 0) // Old browsers

answered Aug 29 '12 at 10:39

pkurek

606
4
13

score 0 · Accepted Answer · answered Aug 29 '12 at 10:49

I implemented something like this.

function Test()
{
}

Test.prototype.setval = function(options)
{
    alert(options);
}

function setclrques(qq,op)
{
    var obj = window['sld_'+qq];
    if( obj!=undefined && obj.setval!=undefined )
    {
        obj.setval(op);
    }
}

var sld_qwer = new Test();
setclrques("qwer","test!");

However I'd recommend you to consider associative array instead of separate variables for storing the objects if possible:

function Test()
{
}

Test.prototype.setval = function(options)
{
    alert(options);
}

var sld = [];
sld["qwer"] = new Test();

function setclrques(qq,op)
{
    var obj = sld[qq];
    if( obj!=undefined && obj.setval!=undefined )
    {
        obj.setval(op);
    }
}

setclrques("qwer","test!");

Great suggestion! I didn't know JavaScript had support for associative arrays. Thanks! — Graphic Equaliser, Aug 29 '12 at 11:28

How to see if a variable name has been defined in Javascript

4 Answers4