This VTL Client Script is an indicator derived from Stochastic with only difference being an extra line called J that represents the %D value from %K.
The %J value can gp beyond [0, 100] for %K and %D lines.
The formula compares the current close to the low, high and range of a set period and then creates two lines, %K and %D.
%K is the faster line, %D is simply a moving average of %K and provides a signal line.
The KDJ adds a third line, the %J, for a total of three; %K%D%J, KDJ. The %J line is nothing more than the difference between the other two lines.