Javascript Generate Unique Number Based on String

function makeid(length) {
    var result           = '';
    var characters       = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
    var charactersLength = characters.length;
    for ( var i = 0; i < length; i++ ) {
      result += characters.charAt(Math.floor(Math.random() * 
 charactersLength));
   }
   return result;
}

console.log(makeid(5));

Create a unique number with javascript time

If you just want a unique-ish number, then

var timestamp = new Date().getUTCMilliseconds();

would get you a simple number. But if you need the readable version, you're in for a bit of processing:

var now = new Date();

timestamp = now.getFullYear().toString(); // 2011
timestamp += (now.getMonth < 9 ? '0' : '') + now.getMonth().toString(); // JS months are 0-based, so +1 and pad with 0's
timestamp += ((now.getDate < 10) ? '0' : '') + now.getDate().toString(); // pad with a 0
... etc... with .getHours(), getMinutes(), getSeconds(), getMilliseconds()

Is there a simple way to convert a unique string of characters into a unique number in JavaScript?

Basically, you want an injective transformation f : S → N, where S is the set of JS strings of length 7 with characters A-Z1-9, and N is the set of all JS numbers.

A possible approach is considering that the strings in S are a positional encoding of numbers, as you attempted.

However, in order to be injective (avoid collisions), you should multiply the value of each character by the basis to the power of the position.

For example, given the following table of character values

0 ⟶  0
1 ⟶  1
⋮      ⋮
9 ⟶  9
A ⟶ 10
B ⟶ 11
⋮      ⋮
Z ⟶ 35

A6HJ92B would become 10×36⁶ + 6×36⁵ + 17×36⁴ + 19×36³ + 9×36² + 2×36 + 11, that is, 22160072099.

You can do the conversion easily with parseInt and toString:

parseInt('A6HJ92B', 36); // 22160072099
(22160072099).toString(36).toUpperCase(); // "A6HJ92B"

If you want to use an arbitrary table of values, you will have to code the transformations manually.

Note that in JS, numbers are double-precision floats of 64 bits. That means there is a finite precision, and you can't store arbitrarily big integers. It won't work properly beyond this maximum

Number.MAX_SAFE_INTEGER; // 9007199254740991
Number.MAX_SAFE_INTEGER.toString(36).toUpperCase(); // "2GOSA7PA2GV"

But since your strings only have 7 characters, it should be enough.

How can I create unique IDs with JavaScript?

could you not just keep a running index?

var _selectIndex = 0;

...code...
var newSelectBox = document.createElement("select");
newSelectBox.setAttribute("id","select-"+_selectIndex++);

EDIT

Upon further consideration, you may actually prefer to use array-style names for your selects...

e.g.

<select name="city[]"><option ..../></select>
<select name="city[]"><option ..../></select>
<select name="city[]"><option ..../></select>

then, on the server side in php for example:

$cities = $_POST['city']; //array of option values from selects

EDIT 2 In response to OP comment

Dynamically creating options using DOM methods can be done as follows:

var newSelectBox = document.createElement("select");
newSelectBox.setAttribute("id","select-"+_selectIndex++);

var city = null,city_opt=null;
for (var i=0, len=cities.length; i< len; i++) {
    city = cities[i];
    var city_opt = document.createElement("option");
    city_opt.setAttribute("value",city);
    city_opt.appendChild(document.createTextNode(city));
    newSelectBox.appendChild(city_opt);
}
document.getElementById("example_element").appendChild(newSelectBox);

assuming that the cities array already exists

Alternatively you could use the innerHTML method.....

var newSelectBox = document.createElement("select");
newSelectBox.setAttribute("id","select-"+_selectIndex++);
document.getElementById("example_element").appendChild(newSelectBox);

var city = null,htmlStr="";
for (var i=0, len=cities.length; i< len; i++) {
    city = cities[i];
    htmlStr += "<option value='" + city + "'>" + city + "</option>";
}
newSelectBox.innerHTML = htmlStr;

I want to generate unique ID

If you're just looking to generate a random sequence according to that pattern, you can do so relatively easily. To ensure it's unique, you'll want to run this function and then match it against a table of previously generated IDs to ensure it has not already been used.

In my example below, I created two functions, getRandomLetters() and getRandomDigits() which return a string of numbers and letters the length of the argument passed to the function (default is length of 1 for each).

Then, I created a function called generateUniqueID() which generates a new ID according to the format you specified. It checks to see if the ID already exists within a table of exiting IDs. If so, it enters a while loop which loops until a new unique ID is created and then returns its value.

const existingIDs = ['AA1111','XY1234'];
const getRandomLetters = (length = 1) => Array(length).fill().map(e => String.fromCharCode(Math.floor(Math.random() * 26) + 65)).join('');
const getRandomDigits = (length = 1) => Array(length).fill().map(e => Math.floor(Math.random() * 10)).join('');
const generateUniqueID = () => {
  let id = getRandomLetters(2) + getRandomDigits(4);
  while (existingIDs.includes(id)) id = getRandomLetters(2) + getRandomDigits(4);
  return id;
};
const newID = generateUniqueID();

console.log(newID);