How to check for undeclared variables when an html element has the same id name?
Question
How do I check whether a variable name is "in use"? Meaning: how do I check if a variable name has already been used in a var varname = 'something' statement?
Normally, I would just check if typeof varname == 'undefined' , which seems to work fine. The issue is when there is an element on the page with id="varname" . Now, when I check typeof varname == 'undefined' , i get false regardless, because varname is some HTML element.
varname is not "in use", but it is not undefined either.
<body id="test1"></body>
<script>
//if <body> has an id of test1, how do i check if test1 is undeclared?
console.log(typeof test1 == 'undefined'); // the <body> element causes this to be true
console.log(typeof test2 == 'undefined'); // outputs true as expected
</script>
Additionally, can you check for the corner case: var test1 = document.getElementById('test1') has been executed?
3 answers
solution1
6 ACCPTED 2013-05-09 05:06:17
The only possible way to achieve what you need is through the evil eval() , as it is not possible to access local variables through their name as strings. (Globals are possible using window["variableNameAsString"] .)
The solution snippet presented below has the following effect:
Returns
trueifvarNamewas declared and initialized (even if withnull) and is readable from the current scope. Otherwise returnsfalse.
Source:
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 < 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;
}
+ ')';
Usage:
To test if a variable with name variableName is defined:
eval(isDefinedEval + '("' + 'variableName' + '")') === true
To check if it is not defined:
eval(isDefinedEval + '("' + 'variableName' + '")') === false
In the fiddle you'll find lots of unit tests demonstrating and verifying the behavior.
Tests:
- Several tests are included in the fiddle;
- This snippet was tested in IE7, IE8 and IE9 plus latests Chrome and Firefox.
Explanation:
The function must be used through eval() because it needs to have access to the variables locally declared.
To access such variables, a function must be declared in the same scope. That's what eval() does there: It declares the function in the local scope and then calls it with varName as argument.
Aside that, the function basically: - Removes the ID attribute of every element that has an ID === varName ; - Checks if the variable is undefined ; - And reassign the ID of those elements it removed the attribute.
Note: this answer was heavily edited, some coments may not be still appliable.
solution2
1 2013-05-09 04:23:06
Not sure if this what you are looking for.
if(typeof test1 == "undefined" && document.getElementById("test1") == null) {
//There is no element with an id="test1" and a variable test1
}
solution3
1 2013-05-09 05:08:51
In a context other than the global context in some browsers, it is impossible to reliably determine if a variable has been defined (either by being declared or otherwise initialised) or not other than using try..catch .
Therefore it's not a good strategy to need to do that.
In a global context, in non–IE browsers, you can do something like:
var global = this;
if (!global.hasOwnProperty('foo')) {
// there is no global foo
}
but IE doesn't implement hasOwnProperty on the global object. Note that DOM element names and IDs do not overwrite the value of declared global variables.
But anyway, all this is useless since you may well have:
var foo = document.getElementById('foo');
What now?
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