What is the reason for getting an error when I use eval() with prompt()?

Question

function nameValidation(){
  var x = eval(prompt("please enter a value"));
  var i = 0;
  while(i < Infinity){
    if(x !== "abcdefghi"){
      alert("please enter a valid name")
      x = eval(prompt("please enter a value"));
    }
    else if (x === "abcdefghi"){
      break
    }
    i++
 }
}
nameValidation()

I am trying to validate a name using eval() and prompt(). When I write any string I get an error:

VM146:1 Uncaught Reference Error: klklk is not defined at eval (eval at nameValidation (Java-Script:7), :1:1)at nameValidation (:7:11)at :15:1.

I tried to use other conversion methods but I got an infinite loop without stop, although I am using break in my code.

function nameValidation(){
  var x = eval(prompt("please enter a value"));
  var i = 0;
  while(i < Infinity){
    if(x !== "abcdefghi"){
      alert("please enter a valid name")
      x = eval(prompt("please enter a value"));
    }
    else if (x === "abcdefghi"){
      break
    }
    i++
 }
}

I expected the program to work without errors, on the other hand, I expect to have the alert: please enter a valid name if the name is not a string and if the name is a string the programme breaks out of the loop.

In that code the eval() evaluates the x, if it is not a string the program will alert "please enter a valid name". That's because I use identically, not equal operator ("!==") to check the type and if x identically equal to string the program breaks out of the loop. My concern is why I receive the error above in my question.

Answer 1

The eval method with prompt does not return anything, and hence, the value of your variable x is undefined. Instead of eval & prompt, you can only use prompt.

function nameValidation(){
  var x = prompt("please enter a value");  //changed this
  var i = 0;
  while(i < Infinity){
    if(x !== "abcdefghi"){
      alert("please enter a valid name")
      x = prompt("please enter a value"); // changed this
    }
    else if (x === "abcdefghi"){
      break
    }
    i++
 }
}

The larger question is that why did you need eval in the first place. Also, your check for i<Infinity and i++ in the loop does not make much sense, as i will never exceed Infinity

Answer 2

As per documentation the eval() function expects a string which you are missing in your code:

eval(string)

where

Parameters

string

A string representing a JavaScript expression, statement, or sequence of statements. The expression can include variables and properties of existing objects.

Return value

The completion value of evaluating the given code. If the completion value is empty, undefined is returned.

Reference Link

You just need to make a simple change in eval as shown below:

/**
 * Just correcting the error in the mentioned code of the question
 */ 
function nameValidation(){
  var x = eval('prompt("please enter a value")');
  var i = 0;
  while(i < Infinity){
    if(x !== "abcdefghi"){
      alert("please enter a valid name")
      x = eval('prompt("please enter a value")');
    }
    else if (x === "abcdefghi"){
      break;
    }
    i++;
 }
}

/**
 * Fixing Eval() issue as mentioned in code as well as checking that the string should not be empty.
 */ 
function nameValidation2(){
    var x = eval('prompt("please enter a value")');
    var i = 0;
    while(i < Infinity){
      if(x == ""){
          alert("please enter a valid name")
          x = eval('prompt("please enter a value")');
      }
      else {
          break;
      }
      i++;
   }
}
nameValidation2();

Answer 3

No need to use eval(), use the Regular_expression to check whether an entered value is not integer only. As per suggestion from t-niese there are the names who contains special characters like ',é, ö still many more which we don't even know about, ex:

Prashant D'Souza
Mary Kate 
André 
Sören
Jørgen
Prashant0123 -- in case of username 

You can also read about Special characters in the name here and Regular_Expressions Guide by Mozilla

function nameValidation() {
 var x = prompt("please enter a value");
 var i = 0;
 var regex = /^[0-9]+$/;

 while (i < Infinity) {
  if (x === "abcdefghi") {
   break;
  } else if (!x.match(regex)) {
   break;
  } else {
   alert("please enter a valid name")
   x = prompt("please enter a value")
  }
  i++;
 }
}
nameValidation();