JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://rawgit.com/WebReflection/0ca8fac8013f76fb4d06/raw/1bdfb5f472f59dfc3ce2855fdd0592265178c26e/Promise-rejet.js"></script>

JavaScript

// sync cancellation - does what I expect
var d = new Promise(function(resolve, reject, c){
    c(function(){
        setTimeout(alert.bind(null, "Cancel"));  
    });
    setTimeout(resolve);
});
d.cancel();
d.cancel();