Random acquisition of 4-digit non repeated verification code

 function getCode() {
    let str = 'qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM0123456789'
    let code = ''
    // while  Execute when the conditions are met , If the conditions are not met, the system will jump out of "do not execute" 
    while (code.length < 4) {
      // Math.round The rounding method 
      //  Math.random  Random access 0-1 Number between 
      // *61  Because the verification code is from a-z,A-Z,1-9 Yes 62 position ,str String from 0 Start calculating , So we need to *61
      let random = Math.round( Math.random() * 61)
      if(code.indexOf(str[random]) === -1) {
        code += str[random]
      }
    }
    return code
  }
  const res = getCode()
  console.log('res', res);