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    <p>The only possible way to achieve what you need is through the evil <code>eval()</code>, as it is not possible to access local variables through their name as strings. (Globals are possible using <code>window["variableNameAsString"]</code>.)</p> <p>The solution snippet presented below has the following effect:</p> <blockquote> <p>Returns <strong><code>true</code></strong> if <strong><code>varName</code></strong> was declared and initialized (even if with <strong><code>null</code></strong>) and is readable from the current scope. Otherwise returns <strong><code>false</code></strong>.</p> </blockquote> <p><strong><a href="http://jsfiddle.net/acdcjunior/8kHNu/" rel="nofollow">DEMO jsFiddle here.</a></strong></p> <h3>Source:</h3> <pre><code>function removeIdOfAllElementsWithId(id) { var element, elementsFound = []; while ((element = document.getElementById(id)) !== null) { element.removeAttribute('id') elementsFound.push(element); } return elementsFound; } function assignIdToElements(elements, id) { for (var i = 0, n = elements.length; i &lt; n; i++) { elements[i].id = id; } } var isDefinedEval = '(' + function (isDefinedEvalVarname) { var isDefinedEvalResult; var isDefinedEvalElementsFound = removeIdOfAllElementsWithId(isDefinedEvalVarname); try { isDefinedEvalResult = eval('typeof '+isDefinedEvalVarname+' !== "undefined"'); } catch (e) { isDefinedEvalResult = false; } assignIdToElements(isDefinedEvalElementsFound, isDefinedEvalVarname); return isDefinedEvalResult; } + ')'; </code></pre> <h1>Usage:</h1> <p>To test if a variable with name <code>variableName</code> is defined:</p> <pre><code>eval(isDefinedEval + '("' + 'variableName' + '")') === true </code></pre> <p>To check if it is not defined:</p> <pre><code>eval(isDefinedEval + '("' + 'variableName' + '")') === false </code></pre> <p>In the fiddle you'll find lots of unit tests demonstrating and verifying the behavior.</p> <h3>Tests:</h3> <ul> <li>Several tests are included in the fiddle;</li> <li>This snippet was tested in IE7, IE8 and IE9 plus latests Chrome and Firefox.</li> </ul> <h3>Explanation:</h3> <p>The function must be used through <code>eval()</code> because it needs to have access to the variables locally declared.</p> <p>To access such variables, a function must be declared in the same scope. That's what <code>eval()</code> does there: It declares the function in the local scope and then calls it with <code>varName</code> as argument.</p> <p>Aside that, the function basically: - Removes the ID attribute of every element that has an ID === <code>varName</code>; - Checks if the variable is <code>undefined</code>; - And reassign the ID of those elements it removed the attribute.</p> <p><sup>Note: this answer was heavily edited, some coments may not be still appliable.</sup></p>
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    1. POHow to check for undeclared variables when an html element has the same id name?
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    1. POHow to check for undeclared variables when an html element has the same id name?
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    1. COCool! This seems to work. Is there any reason `eval("varname")` throws an exception as opposed to `varname`? Just curious.
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      1. USuber5001
    2. COOoops. The first "false" should be "true", since `test1` is declared and therefore exists before any code is executed. It just isn't assigned a value until after the first few tests. Note that `eval('test1')` simply returns the string "test1".
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    3. COAnother good use of evil `eval` but sometimes it's necessary.
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