Regex.test() is giving true false sequential?

Question

Anyone can explain me, why local Regex variable and non local Regex variable have different output.

var regex1 = /a|b/g;
function isAB1() {
    return regex1.test('a');
}
console.log(isAB1()); // true
console.log(isAB1()); // false
console.log(isAB1()); // true
console.log(isAB1()); // false


function isAB2() {
    var regex2 = /a|b/g;
    return regex2.test('a');
}
console.log(isAB2()); // true
console.log(isAB2()); // true
console.log(isAB2()); // true
console.log(isAB2()); // true

I have created a JSFiddle for the same here.

Answer 1

You gave your regex the g flag which means it will globally match results. By doing so you explicitly asked your regex to keep state about its previous matches.

var regex1 = /a|b/g;

> regex1.lastIndex
0
> regex1.test('a');
true
> regex1.lastIndex
1
> regex1.test('a');
false

If you remove the g you will get the results you were expecting.

You can check your expressions .lastIndex property for when it is done matching.