JSFiddle - React, Tailwind, and code Playground

JavaScript

function round(n, what) {
    var i = 0;
    if (n < 1) {
        while(n < 1) {
            n = n*10;
            i++;
        }
    }
    return '0.' + (new Array(i)).join('0') + n.toFixed(what).replace('.','').slice(0,-1);
}

console.log( round(0.0000000000025666, 2) );