For archiving this think you have to do three steps.

First: Collect all values you need into an array.

var myPossibleValues = ["A","B"];

Second: Generating a random integer number.

Based on Generate random number between two numbers in JavaScript

var randNum = Math.round(Math.random() * myPossibleValues.length);

The parameter myPossibleValues.length allows you to exentd the array with additional elements and use it as maximum range value.

Third: Read the generated number, apply it on array and associate it to your data attribute.

 var videoDiv = document.getElementsByClassName("demo");
     videoDiv[0].setAttribute("data-id",myPossibleValues[randNum]);

Due to the fact that getElementsByClassName() returns an array, the [0] is nessecary for the -only- first matched html element.

This answer is a good solution but since the Issue only needs to select a value from two possibilities I would not overcomplicate things.

const demo = document.querySelector('.demo');
demo.setAttribute('data-id', Math.random() > 0.5 ? 'A' : 'B');

What this script does:

• get one element that matches the selector .demo
• set the attribute data-id to either A or B depending on if the random value is above 0.5 or below - the chances are 50/50