Checking if an argument a string

Question

I have a JavaScript function that uses document.getElementById(). I want to upgrade it to be able to use jQuery selectors ($(this).parent().find("blah")), however it needs to be able to use the original method for backwards compatibility. Is there a way I can test if the argument passed to the function is a string (so I can use getElementById) or a jQuery object (not a string).

I could use .length, but is this a failsafe method of determining whether the argument is a string?

As long as I can test for strings, the jQuery branch can just go in an else - I don't need to make absolutely sure it's not a string, although it would be nice to test if it's jQuery too.

Thanks,

James

score 4 · Answer 1 · answered Dec 11 '10 at 10:52

4

following code returns true:

"somestring".constructor == String

answered Dec 11 '10 at 10:52

decyclone

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+1 Better answer than mine as this works for strings converted to Objects too. – David Tang Dec 11 '10 at 11:10

score 3 · Accepted Answer · edited Dec 12 '10 at 18:50

3

Object.prototype.toString.call(your_argument) == "[object String]"

edited Dec 12 '10 at 18:50

Eric

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answered Dec 11 '10 at 10:52

anton_byrna

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Thanks :-) I tried decyclone's answer and had a bit of trouble. Yours works great though. – Bojangles Dec 11 '10 at 11:58
Thanks for the great answer, can you comment on how this compares to using typeof? – Jan 15 '13 at 16:49
@pure_code good explanation http://javascriptweblog.wordpress.com/2011/08/08/fixing-the-javascript-typeof-operator/ – anton_byrna Jan 16 '13 at 15:37

score 2 · Answer 3 · answered Dec 11 '10 at 10:56

2

Is this what you're after?

var str = "blah";
if (typeof str == "string") {

} else {


}

And length is definitely not the way to go. Arrays will also have a length property, not to mention any custom object could as well.

answered Dec 11 '10 at 10:56

David Tang

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+1 for telling me not to use `.length` - thanks for the advice :-) – Bojangles Dec 11 '10 at 11:04

score 0 · Answer 4 · edited May 23 '17 at 10:33

0

I think instanceOf is what you are looking for. See this post: What is the instanceof operator in JavaScript?

edited May 23 '17 at 10:33

Community

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answered Dec 11 '10 at 10:52

Austin Lin

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This will only work for strings converted into an Object, e.g. through `new String("test")`, not primitives. – David Tang Dec 11 '10 at 11:03

score 0 · Answer 5 · answered Dec 01 '17 at 09:38

JavaScript has two kinds of strings. This checks for both kinds.

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

This will throw always a ReferenceError, if the argument is undeclared although typeof would not throw an error, because JavaScript evaluates from left to right (see Short-circuit evaluation).

Checking if an argument a string

5 Answers5