Determining if a variable has been defined in JavaScript

In my JavaScript code, I declare global variables named with the prefix "sld_label" where "label" can be any string. These variables are initialized as objects that contain a function called setval. Now, I have a function that receives a label in a string parameter called qq and I need to determine if the corresponding sld_label variable has been defined. How can I achieve this without causing an error?

Currently, I am using the following code snippet (which works but triggers an error in Firebug):-

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

Any assistance would be greatly appreciated!

javascriptvariables

Answer №1

In the case that it is global, it will become a member of window, allowing the code below to function correctly:

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

Answer №2

I recently developed a similar solution to this problem.

function Example()
{
}

Example.prototype.updateValue = function(options)
{
    alert(options);
}

function setQuestionColor(question, option)
{
    var object = window['sld_'+question];
    if( object!=undefined && object.updateValue!=undefined )
    {
        object.updateValue(option);
    }
}

var sld_qwer = new Example();
setQuestionColor("qwer","test!");

However, I would suggest considering using an associative array instead of individual variables for storing objects:

function Example()
{
}

Example.prototype.updateValue = function(options)
{
    alert(options);
}

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

function setQuestionColor(question, option)
{
    var obj = sld[question];
    if( obj!=undefined && obj.updateValue!=undefined )
    {
        obj.updateValue(option);
    }
}

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

Answer №3

Check if the value of "sld_" + qq is an object, then assign the value of op to it.

Answer №4

To determine if a variable exists:

if (typeof myVariable != 'undefined') // Can be used in any scope
if (window['variableName'] != undefined) // Works in global scope
if (window['variableName'] != void 0) // Compatible with older browsers

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