valid-number.js

/**
 * Valid Number
 *
 * Validate if a given string can be interpreted as a decimal number.
 *
 * Some examples:
 * "0" => true
 * " 0.1 " => true
 * "abc" => false
 * "1 a" => false
 * "2e10" => true
 * " -90e3   " => true
 * " 1e" => false
 * "e3" => false
 * " 6e-1" => true
 * " 99e2.5 " => false
 * "53.5e93" => true
 * " --6 " => false
 * "-+3" => false
 * "95a54e53" => false
 *
 * Note: It is intended for the problem statement to be ambiguous. You should gather all requirements up front before
 * implementing one. However, here is a list of characters that can be in a valid decimal number:
 *
 * Numbers 0-9
 * Exponent - "e"
 * Positive/negative sign - "+"/"-"
 * Decimal point - "."
 * Of course, the context of these characters also matters in the input.
 *
 * Update (2015-02-10):
 * The signature of the C++ function had been updated. If you still see your function signature accepts a const char *
 * argument, please click the reload button to reset your code definition.
 */

/**
 * @param {string} s
 * @return {boolean}
 *
 * Basically the number should match this regular expression:
 * [-+]?(([0-9]+(.[0-9]*)?)|.[0-9]+)(e[-+]?[0-9]+)?
 */
const isNumber = s => {
  s = s.trim();

  // Define some flags
  let pointSeen = false;
  let eSeen = false;
  let numberSeen = false;
  let numberAfterE = true;

  // Scan from left to right
  for (let i = 0; i < s.length; i++) {
    const curr = s[i];

    if ('0' <= curr && curr <= '9') {
      numberSeen = true;
      numberAfterE = true;
    } else if (curr === '.') {
      if (eSeen || pointSeen) {
        return false;
      }
      pointSeen = true;
    } else if (curr === 'e') {
      if (eSeen || !numberSeen) {
        return false;
      }
      eSeen = true;
      numberAfterE = false;
    } else if (curr === '-' || curr === '+') {
      if (i > 0 && s[i - 1] !== 'e') {
        return false;
      }
    } else {
      return false;
    }
  }

  return numberSeen && numberAfterE;
};

export { isNumber };