JavaScript question

Read the input from the document. This string must be between 10 and 20 characters long, inclusive, and consist of only 0s and 1s. If the string does not meet these conditions, output the result 0. If the input is valid then interpret it as a binary number. Using only loops and substring operations calculate the integer value of the binary number. For simplicity, we will assume little endian numbers ... the smallest number is on the left and get bigger moving to the right. Recall that binary values are powers of 2, with the first few values being 20 = 1, 21 = 2, 20 + 21 = 3, etc.

May I have javascript code and HTML code? thanks