JS realizes the function of displaying security bar according to password length

You can often see that the digital password will display the security bar function according to the length of the password entered on 1 website. How is this function realized based on js? Let's look at the implementation code, which is as follows:

// Display security bar according to password length 
          <ul class="clear">
          <li> Password: </li>
          <li> <input type="password" id="pwd" name="pwd" class="in" onKeyUp=pwStrength(this.value) onBlur=pw_y("pwd","pwd1")></li>
          <li><em class="red">*</em></li>
          <li class="i2 grey"><table border="0" bordercolor="#cccccc" style='display:marker'>
          <tr align="center">
            <td id="strength_L" bgcolor="#eeeeee"> Weak </td>
            <td id="strength_M" bgcolor="#eeeeee"> Medium </td>
            <td id="strength_H" bgcolor="#eeeeee"> Strong </td>
          <td align="left" >
           <label id="pwd1"> Use numbers, letters, underscores, and lengths in  6 - 20  Between characters </label></td>
            </tr>
          </table>         
          </li>
          </ul>
function pwStrength(pwd){
  O_color="#eeeeee";
  L_color="#FF0000";
  M_color="#FF9900";
  H_color="#33CC00";
  if (pwd==null||pwd==''){
    Lcolor=Mcolor=Hcolor=O_color;
  }else{
    S_level=checkStrong(pwd);
    switch(S_level) {
      case 0:
        Lcolor=Mcolor=Hcolor=O_color;
      case 1:
        Lcolor=L_color;
        Mcolor=Hcolor=O_color;
        break;
      case 2:
        Lcolor=Mcolor=M_color;
        Hcolor=O_color;
        break;
      default:
        Lcolor=Mcolor=Hcolor=H_color;
      }
  }
  document.getElementById("strength_L").style.background=Lcolor;
  document.getElementById("strength_M").style.background=Mcolor;
  document.getElementById("strength_H").style.background=Hcolor;
  return;
}