Password generator function in javascript

passgen.js

function generate(size) {
    let config = {
      numbers: true,
      symbols: false
    };
    // add symbols if needed
    // config.symbols = true;

    let alphabet_low = "abcdefghijklmnopqrstuvwxyz"
    let alphabet_up = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    let numbers = "0123456789";
    let symbols = "~!@#$%^&*()_+=-;:{}[]<>?/";

    var availChars = alphabet_low + alphabet_up; // can't TURN OFF the alphabet
    if (config.numbers) {
      availChars = alphabet_low + numbers + alphabet_up;
    }
    if (config.symbols) {
      availChars += symbols;
    }

    // A reasonably "intuitive" way to generate a password:
    var charPos;
    var pwChar;
    var pwLength = 10;  // Change for shorter or longer password

    var pw = "";
    for (i = 0; i < pwLength; i++) {
      charPos = Math.floor(Math.random() * availChars.length);
      pwChar = availChars.charAt(charPos);
      pw = pw + pwChar;
    }
    return pw;
}