JSFiddle - React, Tailwind, and code Playground

JavaScript

function roundToSignificantFigures(num, n) {
    
    if(num === 0) {
        return 0;
    }

    var d = Math.ceil(Math.log10(num < 0 ? -num: num));
    var power = n - parseInt(d);

    var magnitude = Math.pow(10, power);
    var shifted = Math.round(num*magnitude);
    alert(shifted/magnitude);
   
    return shifted/magnitude;
}

roundToSignificantFigures(6666666.0412222919999,3);