JavaScript - flag y meaning in regex (sticky flag)

In this short article, we would like to explain flag y meaning in regular expression in JavaScript.

Qucik solution:

When the y (sticky) flag is used, RegExp class matches pattern only when lastIndex property indicates strat of expected sub-string directly.

Practical example:

// ONLINE-RUNNER:browser;

//    indexes:  0  3
//              |  |
//              v  v
const string = 'abcd';

//                pattern
//                  ||
//                  vv
const expression = /bc/y;  // OR:  new RegExp('bc', 'y');

expression.lastIndex = 0;
console.log(expression.test(string));  // false

expression.lastIndex = 1;
console.log(expression.test(string));  // true   // OR:  expression.exec(text)  // ['bc']

expression.lastIndex = 2;
console.log(expression.test(string));  // false

expression.lastIndex = 3;
console.log(expression.test(string));  // false