Why does a RegExp with global flag give wrong results?

Getting under the hood of global flag

Consistent results with lastIndex reset

Using the global flag persist state from one test to another and it causes confusion due to the unpredictability of the results.

In a search operation, lastIndex is automatically updated, but in case of individual discrete tests, either resetting lastIndex is needed or avoiding global flag g for a consistent testing environment.

if (regex.test(str)) {
  // Your pattern has been spotted
}
regex.lastIndex = 0; // Like a memory wipe in sci-fi movies
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The global flag: to omit or not to omit

In some use cases, specially when scanning through the entire string is not expected after the first match, the global flag might be superfluous.

const matches = str.match(/pattern/); // 'g' flag is no more
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Converting these results to boolean is straightforward with the !! truthiness conversion operator:

const isMatch = !!str.match(/pattern/); // Does it match? Yes or no?
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The art of managing trade-offs

You might prefer instantiating a new RegExp each time for one-off use, thereby ensuring no state leakage.

const isMatch = Boolean(new RegExp('pattern').test(str)); // Fresh as a daisy
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However, for scenarios where performance is key, reusing RegExp objects with lastIndex properly managed can be a good trade-off.

// Performance friendly but mind the cognitive load
const isMatchCaseInsensitive = /pattern/i.test(str);
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Notable limitations and precautions

Handling global regex in iterations

When global regex is used inside an iteration like forEach, it may lead to unexpected results if lastIndex property is not handled.

const regex = /pattern/g; // Global, as vast as universe
['str1', 'str2', 'str3'].forEach((str) => {
  console.log(regex.test(str)); // May log "true", "false", "false"
  regex.lastIndex = 0; // Resetting, blink of an eye and everything is new!
});
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Performance vs. simplicity

If performance is a key concern, precompiled RegExp objects can be reused, keeping lastIndex in check. But remember, there is precious readability at stake here!

The truth about test results

Several time, we expect true results consistently when running tests. But with global flag, lastIndex needs to be resetted or not used at all.

const regex = /pattern/gi;
console.log(regex.test('PATTERN')); // true
console.log(regex.test('PATTERN')); // false but why? Ask lastIndex!
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