Is JavaScript test() saving state in the regex? [duplicate]

Question

Open up a browser console and execute the following code:

var foo = /foo/g;

Then,

foo.test("foo") // true

Then,

foo.test("foo") // false

If you continue to execute foo.test("foo"), you will see alternating true/false responses as if the var foo is actually being modified.

Anyone know why this is happening?

Answer 1

Yes, that's how .test() and .exec() work when the regex is g global. They start at the end of the last match.

You can observe the current last index on the regular expression object using the .lastIndex property.

It's a writeable property, so you can reset it to 0 when/if you need. When the regex is run without finding a match, it automatically resets to 0.

Answer 2

The regex keeps the position of the last test. This allows searching of long strings. You can reset this by setting lastIndex = 0;