JSFiddle - React, Tailwind, and code Playground

JavaScript

function truncateDecimals (num, digits) 
{
    var numS = num.toString();
    var decPos = numS.indexOf('.');
    var substrLength = decPos == -1 ? numS.length : 1 + decPos + digits;
    var trimmedResult = numS.substr(0, substrLength);
    var finalResult = isNaN(trimmedResult) ? 0 : trimmedResult;
    
    // adds leading zeros to the right
    if (decPos != -1){
        var s = trimmedResult+"";
        decPos = s.indexOf('.');
        var decLength = s.length - decPos;
       
            while (decLength <= digits){
                s = s + "0";
                decPos = s.indexOf('.');
                decLength = s.length - decPos;
                substrLength = decPos == -1 ? s.length : 1 + decPos + digits;
            };
        finalResult = s;
    }
    return finalResult;
};
console.log(truncateDecimals(-2.1, 3));