Question

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

Answer 1

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).

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

var answer = answer || {};