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1

How to defined a variable in Javascript, if its not defined. I tried:

var str = "answer";
if(eval(str) == undefined)
   eval("var " + str + " = {}");
alert(answer);

but its displaying error: ReferenceError: answer is not defined

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4 Answers 4

up vote 6 down vote accepted

If you have to do it from a name that's in a javascript variable (that isn't known ahead of time), then you can do it like this:

var str = "answer";
if (typeof window[str] == "undefined") {
    window[str] = {};
}

This uses the fact that all global variables are properties of the window object (in a browser).

If you know the name of the variable ahead of time, then you can simply do this:

var answer = answer || {};
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up vote 1 down vote

if (typeof answer == "undefined") var answer = {};

Eval is executed in separate context.

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up vote 0 down vote

You should use typeof with === operator and 'undefined' (to be sure that nobody overwrote undefined variable) to check if variable is undefined and when it is then assign value to it:

if (typeof answer === 'undefined') {
    var answer = 'Foo bar';
}
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This will always result in answer being equal to 'Foo bar', even if it was defined before (for example in the closure). Google 'variable hoisting js' –  undsoft Jun 30 '13 at 14:55
up vote 0 down vote 
if(someUndefinedVariable === undefined){
    var someUndefinedVariable = 'whatever you want' 
}
alert(someUndefinedVariable) //obviously not undefined anymore

EDIT: code below is not working

or if you do not know the variable name at time of writing the code 

var propertyName = 'answer'; //could be generated during runtime
if(window[propertyName] === undefined){
    var window[propertyName] = 'whatever you want';
}
alert(window[propertyName]);

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The first example works. jsfiddle.net/kQYTy However in the second one I jumped to conclusions. –  Martin Hansen Mar 29 '12 at 8:34
    
The reason the first example works, is because the variable is "hoisted" to the top in their context. adequatelygood.com/2010/2/JavaScript-Scoping-and-Hoisting Just if(someUndefinedVariable === undefined){} will fail though, since you don't declare it at all. –  Martin Hansen Mar 29 '12 at 8:41

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