How to do case insensitive string comparison?
The simplest way to do it (if you're not worried about special Unicode characters) is to call toUpperCase:
var areEqual = string1.toUpperCase() === string2.toUpperCase();
EDIT: This answer was originally added 9 years ago. Today you should use localeCompare with the sensitivity: 'accent' option:
function ciEquals(a, b) { return typeof a === 'string' && typeof b === 'string' ? a.localeCompare(b, undefined, { sensitivity: 'accent' }) === 0 : a === b;}console.log("'a' = 'a'?", ciEquals('a', 'a'));console.log("'AaA' = 'aAa'?", ciEquals('AaA', 'aAa'));console.log("'a' = 'á'?", ciEquals('a', 'á'));console.log("'a' = 'b'?", ciEquals('a', 'b'));The { sensitivity: 'accent' } tells localeCompare() to treat two variants of the same base letter as the same unless they have different accents (as in the third example) above.
Alternatively, you can use { sensitivity: 'base' }, which treats two characters as equivalent as long as their base character is the same (so A would be treated as equivalent to á).
Note that the third parameter of localeCompare is not supported in IE10 or lower or certain mobile browsers (see the compatibility chart on the page linked above), so if you need to support those browsers, you'll need some kind of fallback:
function ciEqualsInner(a, b) { return a.localeCompare(b, undefined, { sensitivity: 'accent' }) === 0;}function ciEquals(a, b) { if (typeof a !== 'string' || typeof b !== 'string') { return a === b; } // v--- feature detection return ciEqualsInner('A', 'a') ? ciEqualsInner(a, b) : /* fallback approach here */;}Original answer
The best way to do a case insensitive comparison in JavaScript is to use RegExp match() method with the i flag.
When both strings being compared are variables (not constants), then it's a little more complicated 'cause you need to generate a RegExp from the string but passing the string to RegExp constructor can result in incorrect matches or failed matches if the string has special regex characters in it.
If you care about internationalization don't use toLowerCase() or toUpperCase() as it doesn't provide accurate case-insensitive comparisons in all languages.
As said in recent comments, string::localeCompare supports case insensitive comparisons (among other powerful things).
Here's a simple example
'xyz'.localeCompare('XyZ', undefined, { sensitivity: 'base' }); // returns 0And a generic function you could use
function equalsIgnoringCase(text, other) { return text.localeCompare(other, undefined, { sensitivity: 'base' }) === 0;}Note that instead of undefined you should probably enter the specific locale you are working with. This is important as denoted in the MDN docs
in Swedish, ä and a are separate base letters
Sensitivity options
Browser support
As of time of posting, UC Browser for Android and Opera Mini do not support locale and options parameters. Please check https://caniuse.com/#search=localeCompare for up to date info.