The ignoreCase accessor property of RegExp instances returns whether or not the i flag is used with this regular expression.
Description
RegExp.prototype.ignoreCase has the value true if the i flag was used; otherwise, false. The i flag indicates that case should be ignored while attempting a match in a string. Case-insensitive matching is done by mapping both the expected character set and the matched string to the same casing.
If the regex is Unicode-aware, the case mapping happens through simple case folding specified in CaseFolding.txt. The mapping always maps to a single code point, so it does not map, for example, ß (U+00DF LATIN SMALL LETTER SHARP S) to ss (which is full case folding, not simple case folding). It may however map code points outside the Basic Latin block to code points within it — for example, ſ (U+017F LATIN SMALL LETTER LONG S) case-folds to s (U+0073 LATIN SMALL LETTER S) and K (U+212A KELVIN SIGN) case-folds to k (U+006B LATIN SMALL LETTER K). Therefore, ſ and K can be matched by /[a-z]/ui.
If the regex is Unicode-unaware, case mapping uses the Unicode Default Case Conversion — the same algorithm used in String.prototype.toUpperCase. For example, Ω (U+2126 OHM SIGN) and Ω (U+03A9 GREEK CAPITAL LETTER OMEGA) are both mapped by Default Case Conversion to themselves but by simple case folding to ω (U+03C9 GREEK SMALL LETTER OMEGA), so "ω" is matched by /[\u2126]/ui and /[\u03a9]/ui but not by /[\u2126]/i or /[\u03a9]/i. This algorithm prevents code points outside the Basic Latin block to be mapped to code points within it, so ſ and K mentioned previously are not matched by /[a-z]/i.
The set accessor of ignoreCase is undefined. You cannot change this property directly.
Examples
Using ignoreCase
const regex = /foo/i;
console.log(regex.ignoreCase); // true