How does ASP implement random letter verification codes?

You make a picture of random letters, and then use numbers to name it. Use the random function of javascript and add 26 rounded letters to multiply the random function by 100. After this selection The number, as the link address of the picture, is ok. A Javascript example of randomly generating pictures (using Math.random() and Math.round methods). This Javascript example explains: First, use Math.random() to get a value greater than or equal to 0, less than A random number of 1; then multiply the random number by 10 to obtain a value greater than or equal to 0 and less than or equal to 10; then use Math.round to round to obtain the closest integer, which is in the range of greater than or equal to 0 and less than or equal to 10; Then combine this random integer with HTML code to randomly generate different pictures.