observing-last-index-values.js

observing-last-index-values.js

function matchAllButUpdateOriginalRegExp(regExp, string) {
  return regExp[Symbol.matchAll].call({
    constructor: { [Symbol.species]: function() { return regExp; } },
    flags: regExp.flags,
    lastIndex: regExp.lastIndex
  }, string);
}

const pattern = /./g;

console.group('Ordinary behavior');

for (let match of 'abc'.matchAll(pattern)) {
  console.log(match, pattern.lastIndex);
}

console.groupEnd();
console.group('Subversive shenanigans');

for (let match of matchAllButUpdateOriginalRegExp(pattern, 'abc')) {
  console.log(match, pattern.lastIndex);
}

console.groupEnd();