Why cesium use when.js instead of Promise of ES6

why cesium use when.js instead of Promise of ES6?
and the version of when.js used in cesium is 1.7.1(it’s too old)
would you change the version of when.js?
or change to use Promise of ES6 in the future

Welcome to the Cesium community!

There’s been some work done here on using native promises and replacing When.js: https://github.com/CesiumGS/cesium/pull/8525. So I’d say yes this is planned. If you’d like to help out, it looks like that PR needs to have the unit tests updated to use native promises. That could be a very helpful contribution.