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Editorial Team
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Asked: 2026-05-30T23:47:52+00:00

Simplest fool proof(?) method to check for string object I’ve seen. Tested with many

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Simplest fool proof(?) method to check for string object I’ve seen. Tested with many different objects types. Are there parameters/circumstances that might trip it up, and are there simper functions available?

function isString(o){
    if(o == null || o == undefined){
        return false;
    }
    if(typeof(o) == 'string'){
        return true;
    }
    if(typeof(o) == 'object'&& typeof(o.valueOf) == 'function' && typeof(o.valueOf()) == 'string'){
        return true;
    }
    return false;
}

Here’s what I’ve check with:

<html>
<body>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4/jquery.min.js"></script>
<input id="el">
<script type="text/javascript">

function isString(o){
    if(o == null || o == undefined){
        return false;
    }
    if(typeof(o) == 'string'){
        return true;
    }
    if(typeof(o) == 'object'&& typeof(o.valueOf) == 'function' && typeof(o.valueOf()) == 'string'){
        return true;
    }
    return false;
}
function tellIfString(o){
    if(isString(o)){
        return "A string!<br />";
    }else{
        return "Not a string!<br />";
    }
}
function CustomO(a){
    this.a = a;
}
literal = 'string';
objectWrapped = new String('why do i feel different?');
stringNumber = '345';
number = 234;
numberWrapped = new Number(3345);
object = {};
array = [];
bool = true;
nan = 1*'sdf';
regex = /woop/;
regex_o = new RegExp('asasd');
o_no_valueOf = {};
o_no_valueOf = delete(o_no_valueOf.valueOf);
customO = new CustomO({});
document.write("literal Is "+tellIfString(literal));
document.write("objectWrapped string Is "+tellIfString(objectWrapped));
document.write("stringNumber Is "+tellIfString(stringNumber));
document.write("number Is "+tellIfString(number));
document.write("numberWrapped Is "+tellIfString(numberWrapped));
document.write("object Is "+tellIfString(object));
document.write("array Is "+tellIfString(array));
document.write("bool Is "+tellIfString(bool));
document.write("nan Is "+tellIfString(nan));
document.write("null Is "+tellIfString(null));
//document.write("notdefined Is "+tellIfString(notdefined));//don't need to check variables that haven't been defined, they error before they get to the function
document.write("undefined Is "+tellIfString(undefined));
document.write("Math Is "+tellIfString(Math));
document.write("function Is "+tellIfString(function(){}));
document.write("regex Is "+tellIfString(regex));
document.write("regexO Is "+tellIfString(regex_o));
document.write("o_no_valueOf Is "+tellIfString(regex_o));
document.write("customO Is "+tellIfString(customO));
document.write("jQuery return Is "+tellIfString($('#el')));
document.write("document Is "+tellIfString(document));//this case is important because it requires the typeof(o.valueOf) == 'function'
document.write("window Is "+tellIfString(window));
document.write("document.cookie Is "+tellIfString(document.cookie));
document.write("window.open Is "+tellIfString(window.open));
document.write("window.location Is "+tellIfString(window.location));
document.write("All objects evaluated!");

</script>

</body>
</html>

See also(especially the parts about new String and .valueOf()
https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/String

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1 Answer

  1. Editorial Team

    I think it’s more sensible to write:

    function isString(o)
    { return typeof(o) == 'string' || o instanceof String; }

    Edited to add: This is not absolutely foolproof; Felix Kling, in the comments, points out one case where it will not work. Based on this page, I believe that this:

    function isString(o)
    { return Object.prototype.toString.call(o) === '[object String]'; }

    is “more” foolproof. (I’ve found that you can still break it by messing with Object.prototype.toString, and perhaps in other ways, but if you distrust your own code that much, then there’s really nothing you can do.)

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