index.ts

// HELP:

export const isAdditiveNumber = (num: string) => {
  // The first two numbers
  let a = 0
  let b = 0

  // Length of the first number
  for (let i = 0; i < num.length / 2; i++) {
    // Last digit of the first number
    const _a = num.charCodeAt(i) - 48

    // Second number starts with zero?
    const zero = num.charCodeAt(i + 1) === 48

    // Update the first number
    a = a * 10 + _a

    // Reset the second number
    b = 0

    // Length of the second number
    for (var j = 1; j <= num.length / 2; j++) {
      // Invalid number
      if (zero && j > 1) continue

      // Update the second number
      b = b * 10 + (num.charCodeAt(i + j) - 48)

      // Validate
      if (validate(a, b, num)) return true
    }
  }

  return false
}

const validate = function(a: number, b: number, target: string) {
  const limit = target.length
  let str = '' + a + b

  // No place for the third number
  if (str.length >= limit) return false

  // Generate sequence
  while (str.length < limit) {
    // Append number
    const c = a + b
    str += c

    // Validate
    if (str === target) return true

    // Swap
    a = b
    b = c
  }

  return false
}