function isPrime(num){
  // loop numbers to see if any num is divisble by that number
  
  if (num < 2){ return false
    
  } else if (num === 2) {
    return true
  }
  
  for (let i = 2; i < num; i  ) {
    if (num % i === 0) {
      return false
    } else {
      return true
    }
}
}

Tried changing the conditional of the for loop but still comes out as true.

CodePudding user response：

Wait until the whole loop finishes before returning true, otherwise you only check the first iteration and make a decision right away whether the input is prime before checking all the rest of the numbers. Also surely you don't need to iterate the entire length of the number, Math.ceil(num/2) 1 should suffice. This is because there's no point to ever check if anything larger than half of a number will factor into that number. (This will make a big impact on larger numbers)

function checkPrime(){
    var num = document.getElementById("num").value;
    var result = document.getElementById("result");
    var arr=[];
    if (num.length===0)
        result.innerHTML = "Please, specify a number.";
    else if (num<0)
        result.innerHTML = "Please, specify a positive number.";
    else
        result.innerHTML = isPrime(num).toString();
        
    return false;
}

function isPrime(num){
  // loop numbers to see if any num is divisble by that number
  
  if (num < 2)
      return false;
  else if (num === 2)
    return true;
  
  const max = Math.ceil(num/2)   1;
  for (let i = 2; i < max; i  ) {
    if (num % i === 0)
      return false;
  }

  return true;
}

<form onsubmit=" return checkPrime()">
      <label>Enter a number check prime</label>
      <input type="number" id="num" name="num"><br>
      <input type="submit" value="Submit" id="submit">
      <div id="result"></div>
</form>

CodePudding user response：

This means you are testing any number entered to return a bool (true/false) if it is a prime number.

I would modify your code a little bit to give this function a correct answer.

function isPrime(num){
  // loop numbers to see if any num is divisble by that number
  
  if (num < 2){ 
      return false
  }else if(num==2){
      return true;
  }else{
      for (let i = 2; i < num; i  ) {
           if (num % i == 0) {
             return false;
             break;
            }
       }
      return true;
  }
  
}