Is ''.charAt(0) reliably ''?

Question

I know that there are better ways to check for an empty string—bear with me.

This is not checking for empty string in general. I'm actually doing something like:

var s = /*who knows?*/;
switch (s.charAt(0)) {
    // ...
}

and wanted to know if I can avoid having to do an extra if (!s).

Duplicate of http://stackoverflow.com/questions/154059/what-is-the-best-way-to-check-for-an-empty-string-in-javascript — Thunraz, Nov 23 '11 at 15:27
Check out this question: http://stackoverflow.com/questions/154059/what-is-the-best-way-to-check-for-an-empty-string-in-javascript — piebie, Nov 23 '11 at 15:28
Just one tip: `charAt` will fail when `s` is not of type string, other than that it's 100% "safe" as Mike said in his answer. — Shadow The GPT Wizard, Nov 23 '11 at 15:32

Mike Samuel · Accepted Answer · 2011-11-23T15:34:48.770

Yes, as long as the value is a valid string. Section 15.5.4.4 of the language spec says

Let position be ToInteger(pos).

Let size be the number of characters in S.

If position < 0 or position ≥ size, return the empty String.

That was not changed since ES3. Interpreters are pretty good around string function compatibility.

That said, I recently implemented a peephole optimization for charAt to Closure compiler but I did not optimize out of bounds checks because the compiler tends not to optimize what are considered programmer errors.

score 3 · Answer 2 · answered Nov 23 '11 at 15:27

Yes, it will always return "" if the index exceeds the length of the string.

Taken from MDC's documentation of the charAt() function:

Characters in a string are indexed from left to right. The index of the first character is 0, and the index of the last character in a string called stringName is stringName.length - 1. If the index you supply is out of range, JavaScript returns an empty string.

Alex Turpin · Answer 3 · 2011-11-23T15:33:35.183

2

To answer the modified question (or what I think the question is):

If your variable is undefined or not of type string, then .charAt will fail.

Do something like

if (typeof s == "string") {
    switch(s) {
        //...
    }
}

edited Nov 23 '11 at 15:33

answered Nov 23 '11 at 15:26

Alex Turpin

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Or using the `===` operator to avoid type conversions. (An empty string will also compare equal to false, empty list, zero, and whatever else given the weird conversion rules.) – millimoose Nov 23 '11 at 15:28
`switch ("string" === typeof s && s.charAt(0))` will also work. The `default` case will fire when `s` is not a string. – Mike Samuel Nov 23 '11 at 19:23

Is ''.charAt(0) reliably ''?

3 Answers3