For example: To generate 8 unique random numbers and store them to an array, you can simply do this:

var arr = [];
while(arr.length < 8){
    var r = Math.floor(Math.random() * 100) + 1;
    if(arr.indexOf(r) === -1) arr.push(r);
}
console.log(arr);

There are many examples around internet. You can start with using Math.random() . For example: generate random number between 1 and 100

Math.floor((Math.random() * 100) + 1);

Just keep in mind that it is not truly random; it is not cryptographically secure. You should probably look into libraries if you need that

The shortest way to create a number that you can be pretty sure will be unique among as many separate instances as you can think of is

Date.now() + Math.random()

If there is a 1 millisecond difference in function call, it is 100% guaranteed to generate a different number. For function calls within the same millisecond you should only start to be worried if you are creating more than a few million numbers within this same millisecond, which is not very probable.

For more on the probability of getting a repeated number within the same millisecond see https://stackoverflow.com/a/28220928/4617597

Use the basic Math methods:

• Math.random() returns a random number between 0 and 1 (including 0, excluding 1).
• Multiply this number by the highest desired number (e.g. 10)

• Round this number downward to its nearest integer

  Math.floor(Math.random()*10) + 1
  

Example:

//Example, including customisable intervals [lower_bound, upper_bound)
var limit = 10,
    amount = 3,
    lower_bound = 1,
    upper_bound = 10,
    unique_random_numbers = [];

if (amount > limit) limit = amount; //Infinite loop if you want more unique
                                    //Natural numbers than exist in a
                                    // given range
while (unique_random_numbers.length < limit) {
    var random_number = Math.floor(Math.random()*(upper_bound - lower_bound) + lower_bound);
    if (unique_random_numbers.indexOf(random_number) == -1) { 
        // Yay! new random number
        unique_random_numbers.push( random_number );
    }
}
// unique_random_numbers is an array containing 3 unique numbers in the given range

If the number only needs to be unique on the client side, it's possible. You could create a number based off the current time for example:

var id = new Date().getTime();

However, if this unique ID needs to be unique for every client, as the comments in your OP state, you'll need a server-sided solution:

var id = "<?php echo uniqid() ?>";

When creating a random number, only use one that isn't yet in your array:

var number = [];
for(i= 0; i <= 5; i++){
    var num;
    while ( number.includes( num = Math.floor(Math.random()*40) ) );
    number[i] = num;
}
console.log(number);

The empty while loop repeatedly selects a number until it finds one that isn't in the number array. Only after that successfully finishes does it then add that number to the array.

This might be your solution:

function randomNumericChar(lastNumber) {
    var num = Math.floor((Math.random() * 10) + 1);

    if (lastNumber !== null && num === lastNumber) {
        return randomNumericChar(lastNumber);
    } else {
        return num;
    }
}

function createNumber(n) {
    var randomNumber = '';
    var lastChar     = null;

    while (randomNumber.length < n) {
        var num = randomNumericChar(lastChar);

        if (((randomNumber.length + 1) % 2) === 1) {
            lastChar = num;
        }

        randomNumber += num;
    }

    return randomNumber;
}

var randomNumber = createNumber(10);

Edit: forgot about the fact that there might be 1 consecutive number.