Is it possible in Javascript to determine if a certain string is a reserved language keyword such as switch, if, function, etc.? What I would like to do is escaping reserved identifiers in dynamically generated code in a way that doesn't break on browser-specific extensions. The only thought ing to my mind is using eval in a try-catch block and check for a syntax error. Not sure how to do that though. Any ideas?

Is it possible in Javascript to determine if a certain string is a reserved language keyword such as switch, if, function, etc.? What I would like to do is escaping reserved identifiers in dynamically generated code in a way that doesn't break on browser-specific extensions. The only thought ing to my mind is using eval in a try-catch block and check for a syntax error. Not sure how to do that though. Any ideas?

• javascript
• keyword
• reserved-words

Share Improve this question Follow edited Apr 23, 2013 at 4:46 GOTO 0 asked Apr 22, 2013 at 22:10 GOTO 0GOTO 0 48.1k2525 gold badges139139 silver badges164164 bronze badges 2

• 5 I'd just go here, grab all the reserved words put them in an array and check the string. – elclanrs Commented Apr 22, 2013 at 22:12
• What's the context of this question? Are you dynamically creating variable names, or just want to make sure when coding that you don't accidentally use a reserved word (in which case, using an IDE or something like Notepad++ will do). Or use jsLint to check for this stff. – David Gilbertson Commented Apr 23, 2013 at 1:02

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3 Answers 3

Sorted by: Reset to default 6

One option would be to do:

var reservedWord = false;
try {
  eval('var ' + wordToCheck + ' = 1');
} catch {
  reservedWord = true;
}

The only issue will be that this will give false positive for words that are invalid variable names but not reserved words.

As pointed out in the ments, this could be a security risk.

I guess you could solve it using eval, but that seems like sort of a hack. I would go for just checking against all reserved words. Something like this:

var reservedWords = [
    'break',
    'case',
    ...
];

function isReservedWord(str) {
    return !!~reservedWords.indexOf(str);
}

Here is a list of all reserved words: https://developer.mozilla/en-US/docs/JavaScript/Reference/Reserved_Words

Also, a problem with the eval-approach is that some browsers sometimes allows you to use some reserved words as identifiers.

FYI, Some words throws error only in strict mode

ex: package

eval('var package = 1') // No error

'use strict'
eval('var package = 1') // throws error