JSFiddle - React, Tailwind, and code Playground

JavaScript

function getFlooredFixed(v, d) {
    return (Math.floor(v * Math.pow(10, d)) / Math.pow(10, d)).toFixed(d);
}

function cutNumber(number, digitsAfterDot) {
  const str = `${number}`;

  return str.slice(0, str.indexOf('.') + digitsAfterDot + 1);
}

const x = 1.305191185185195;

// getFlooredFixed() test
let start = Date.now();

for (var i = 0; i < 100000; i++) {
  getFlooredFixed(x, 5) 
}

console.log('getFlooredFixed(): ', Date.now() - start + 'ms');

// cutNumber() test
start = Date.now();

for (var i = 0; i < 100000; i++) {
	cutNumber(x, 5);
}

console.log('cutNumber(): ', Date.now() - start + 'ms');