Why is the JavaScript RegExp.test() method behaving as a toggle?

Question

Can anyone explain why the alert() in the following JavaScript code is firing? It appears to be a bug in the RegExp.test() method which reverses its previous decision each time you run the method. I'm using IE7.

I've found a replacement that appears solid, using the string.search(regex) method instead. But, I'm curious whether anyone knows something about this.

  var styleHasWidthRegex = /\bwidth\s*\:/ig;
  var styleText = "WIDTH: 350px";
  var result1 = styleHasWidthRegex.test(styleText);
  var result2 = !styleHasWidthRegex.test(styleText);
  if (result1 == result2) {
    alert("This should never happen!");
  }

score 46 · Accepted Answer · edited Jun 20 '20 at 09:12

46

Your regex has the global (g) flag set. Every time it is executed, it'll update an internal index (the lastIndex property) specifying where it left off, and begin searching at that point the next time through.

Of course, you don't really want that - you want it to start at the beginning every time. So ditch the g flag.

See also: Inconsistent javascript logic behavior

edited Jun 20 '20 at 09:12

Community

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answered May 21 '09 at 19:36

Shog9

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Thanks for making sense out of that. – John Fisher May 21 '09 at 19:44
no problem, John. Took me a while to get it the first time i saw it too! – Shog9 May 21 '09 at 19:49
....you're awesome! Learn something new today :D – afreeland Apr 30 '15 at 19:28
came here to upvote. Thanks! – Vikram Jul 15 '15 at 15:44

score 0 · Answer 2 · answered May 21 '09 at 19:37

0

In this scenario, you should be needing a global tag anyways, since in css declarations a property should only be declared once.

answered May 21 '09 at 19:37

Dmitri Farkov

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Why is the JavaScript RegExp.test() method behaving as a toggle?

2 Answers2

See also: Inconsistent javascript logic behavior