Regular expression test (), using g__ regular expressions with caution

Today, when this expression is presented, a problem is found:
Test () detects whether a specified string exists and returns a Boolean value

var reg=/cat/g;
var str= ' This a cat,this a dog ';
document.write (Reg.test (s));
document.write (Reg.test (str));
According to the reason two times print out should be true,true, and the end result is true,false.
At this point we need to be aware that the search is added to the regular expression we defined, and that G represents the Full-text lookup. And there is a lastindex within the regular expression to record the matching position, and after the first call to test (), then the lastindex is no longer equal to 0, but 10, and the next time the method is called, the string match is matched from the lastindex position. So the end returns false. So don't add g at random.
How to resolve this situation:
1. Remove G;
2. Before using the second time, set reg.lastindex=0.