简单化

const trunc = (n, decimalPlaces) => {
  const decimals = decimalPlaces ? decimalPlaces : 2;
  const asString = n.toString();
  const pos = asString.indexOf('.') != -1 ? asString.indexOf('.') + decimals + 1 : asString.length;
  return parseFloat(n.toString().substring(0, pos));
};

console.log(trunc(3.14159265359));
console.log(trunc(11.1111111));
console.log(trunc(3));
console.log(trunc(11));
console.log(trunc(3.1));
console.log(trunc(11.1));
console.log(trunc(3.14));
console.log(trunc(11.11));
console.log(trunc(3.141));
console.log(trunc(11.111));