JSFiddle - React, Tailwind, and code Playground

JavaScript

function sigfig(n, sf) {
    sf = sf - Math.floor(Math.log(n) / Math.LN10) - 1;
    sf = Math.pow(10, sf);
    n = Math.round(n * sf);
    n = n / sf;
    return n;
}

function precision(n, dp) {
    dp = Math.pow(10, dp);
    n = n * dp;
    n = Math.round(n);
    n = n / dp;
    return n;
}

function nearest(n, v) {
    n = n / v;
    n = Math.round(n) * v;
    return n;
}

alert( sigfig(1379.829, 2) );

alert( precision(1379.829, 2) );

alert( nearest(1379.829, 500) );