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
Add a comment
|
4 Answers
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 || {};
Comments
if (typeof answer == "undefined") var answer = {};
Eval is executed in separate context.
Comments
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';
}
1 Comment
undsoft
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'
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]);
2 Comments
Martin Hansen
The first example works. jsfiddle.net/kQYTy However in the second one I jumped to conclusions.
Martin Hansen
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.