How to get time in milliseconds since the unix epoch in Javascript?

This will do the trick :-

new Date().valueOf() 

Date.now() returns a unix timestamp in milliseconds.

const now = Date.now(); // Unix timestamp in milliseconds
console.log( now );

Prior to ECMAScript5 (I.E. Internet Explorer 8 and older) you needed to construct a Date object, from which there are several ways to get a unix timestamp in milliseconds:

console.log( +new Date );
console.log( (new Date).getTime() );
console.log( (new Date).valueOf() );