JavaScript regex - null argument makes the regex match

Question

I'm writing a regex to be used with JavaScript. When testing I came across some strange behavior and boiled it down to the following:

/^[a-z]/.test("abc"); // <-- returns true as expected
/^[a-z]/.test(null);  // <-- returns true, but why?

I was assuming that the last case was going to return false since it does not meet the regex (the value is null and thus, do no start with a character in the range). So, can anyone explain me why this is not the case?

If I do the same test in C#:

var regex = new Regex("^[a-z]");
var res = regex.IsMatch(null); // <-- ArgumentNullException

... I get an ArgumentNullException which makes sense. So, I guess when testing a regex in JavaScript, you have to manually do a null check?

I have tried searching for an explanation, but without any luck.

Denys Séguret · Accepted Answer · 2014-12-08 13:51:39Z

13

That's because test converts its argument : null is converted to the "null" string.

You can check that in the console :

/^null$/.test(null)

returns true.

The call to ToString(argument) is specified in the ECMAScript specification (see also ToString).

answered Dec 8, 2014 at 13:51

Denys Séguret

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5 Comments

Troels Larsen Over a year ago

It makes me very sad that this is the right answer.

Amit Joki Over a year ago

@TroelsLarsen for more, see wtfjs.com

Lasse Christiansen Over a year ago

Spot on, thanks! This also makes me panda sad, but I guess that's the way it is. While @AmitJoki answer also addresses my question, I'll mark yours as the accepted answer because you were first to respond and has included links to the spec. Thanks.

Amit Joki Over a year ago

@LasseChristiansen-sw_lasse I love Pandas! So don't be sad and go on! :)

Gianluca Ghettini Over a year ago

this is the quintessential Javascript behavior

Amit Joki · Accepted Answer · 2014-12-08 13:58:23Z

9

Here null is getting typecasted to String form which is "null".

And "null" matches your provided regex which is why it is evaluating to true

In Javascript, everything(or mostly) is an Object which has ToString method which will be automatically called upon internally in case there is a need for a typecast.

edited Dec 8, 2014 at 13:58

answered Dec 8, 2014 at 13:52

Amit Joki

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