erc20-decimals-conversion.js

function isString(s) {
  return (typeof s === 'string' || s instanceof String)
}

export function toBaseUnit(value, decimals, BN) {
  if (!isString(value)) {
    throw new Error('Pass strings to prevent floating point precision issues.')
  }
  const ten = new BN(10);
  const base = ten.pow(new BN(decimals));

  // Is it negative?
  let negative = (value.substring(0, 1) === '-');
  if (negative) {
    value = value.substring(1);
  }

  if (value === '.') { 
    throw new Error(
    `Invalid value ${value} cannot be converted to`
    + ` base unit with ${decimals} decimals.`); 
  }

  // Split it into a whole and fractional part
  let comps = value.split('.');
  if (comps.length > 2) { throw new Error('Too many decimal points'); }

  let whole = comps[0], fraction = comps[1];

  if (!whole) { whole = '0'; }
  if (!fraction) { fraction = '0'; }
  if (fraction.length > decimals) { 
    throw new Error('Too many decimal places'); 
  }

  while (fraction.length < decimals) {
    fraction += '0';
  }

  whole = new BN(whole);
  fraction = new BN(fraction);
  let wei = (whole.mul(base)).add(fraction);

  if (negative) {
    wei = wei.mul(negative1);
  }

  return new BN(wei.toString(10), 10);
}