Having some trouble with this operation, found that you can use string replacement syntax to also split a string to array:
NPM_VER=$(npm -v) # 6.12.1
NPM_VER=(${NPM_VER//./ }) # Split on '.' as delimiter
echo "${NPM_VER[0]}" # 6
echo "${NPM_VER[1]}" # 12
echo "${NPM_VER[2]}" # 1