too long

Im trying to get data from my php file using success function(data)

When im using console.log(data) i get; true/false/exists But i cant make a if statement, if i do it dosent alert!!

Heres my js file

function ajaxCall(username, email, password){

$.ajax({
    type: 'POST',
    url: '../register.php',
    data:   {
        'username' : username,
        'email' : email,
        'password' : password,
    },
    success: function(data) {
     if(data === "exists")
     alert("user exists");
    }
});

}

heres my .php file

<?php

$servername = "#";
$username = "#";
$password = "#";
$dbname = "#";

$regusername = $_POST['username']; 
$email = $_POST['email']; 
$regpassword = $_POST['password'];   

$mysqli = new mysqli($servername, $username, $password, $dbname);
$selectQuery = "SELECT username FROM users WHERE username = '$regusername'";
$select = $mysqli->query($selectQuery);
// var_dump($select->num_rows);

if( $select->num_rows == 0) {
    $insertQuery = "INSERT INTO users (username, password, email) VALUES ('$regusername','$regpassword','$email');";
    $insert = $mysqli->query($insertQuery);

        if( $insert == true) {
            echo "true";
        }else {
            echo "false";
        }
}else {
    echo "exists";
}

?>

Use a properly structured data format, such as json:

php:

$data=['exists'=>false];
if( $select->num_rows == 0) {
    $insertQuery = "INSERT INTO users (username, password, email) VALUES ('$regusername','$regpassword','$email');";
    $data['inserted'] = (bool) $mysqli->query($insertQuery);
}else {
    $data['exists']=true;
}
header('Content-Type: application/json');
echo json_encode($data);
die();

JS

$.ajax({
    type: 'POST',
    url: '../register.php',
    data:   {
        'username' : username,
        'email' : email,
        'password' : password,
    },
    success: function(data) {
       if(data.exists){
           alert("user exists");
       }elseif(data.inserted){
           alert("inserted");
       }else{
           alert("did not insert");
       }
});

if(data == "exists") instead of  if(data === "exists")

Chances are there is extra whitespace in the response

Try:

if($.trim(data) === "exists")

You are not responding with an answer to the front, you just echoing 'exist' word in your php handler. Try using json_encode, because the dataType is "json" by default not "html"

Lets's say

if($insert) { 
 $response['exist_status'] = 'exist' 
}else{ 
 $response['exist_status'] = 'do not exist' 
} 
echo json_encode($response);

In your jquery

success: function(response) { 
 if(response.exist_status && response.exist_status == 'exist')  { 
  alert(1); 
 } 
}

I think its better to work with HTTP-Status codes like 404 or 403 (https://en.wikipedia.org/wiki/List_of_HTTP_status_codes) instead of retuning a string. This can by done with PHP (http://php.net/manual/de/function.http-response-code.php) and use the error callback of your ajax call (http://api.jquery.com/jquery.ajax/ --> StatusCode)