The easiest is maybe this:
const time = "125657".replace(/(..?)(..)(..)/, "$1:$2:$3");
const date = "20190702".replace(/(....)(..)(..)/, "$2/$3/$1");
console.log(date, time);The question mark in the first pattern could serve if the time string could have 5 digits instead of 6. If you are certain you always get 6 digits, you can leave it out.