Similarity calculator for string inputs implemented in TypeScript

similarity-calculator.ts

/**
 * Get the similarity percentage between two strings.
 * Based on @link{https://gist.github.com/sumn2u/0e0b5d9505ad096284928a987ace13fb#file-jaro-wrinker-js}
 */
export const calculateStringsSimilarity = (
  firstString: string,
  secondString: string,
  config?: {
    /**
     * If true, the function will return the similarity percentage as a float number between 0 and 1.
     * If false, the function will return the similarity percentage as an float between 0 and 100.
     */
    asRatio?: boolean
  }
) => {
  let matchesFound = 0

  if (firstString.trim() === secondString.trim()) return config?.asRatio ? 1 : 100

  const range =
    Math.floor(Math.max(firstString.length, secondString.length) / 2) - 1
  const matchesInFirstString = new Array(firstString.length)
  const matchesInSecondString = new Array(secondString.length)

  new Array(firstString.length)
    .fill(null)
    .forEach((_, firstStringCharIndex) => {
      const high =
        firstStringCharIndex + range <= secondString.length
          ? firstStringCharIndex + range
          : secondString.length - 1

      let low = firstStringCharIndex >= range ? firstStringCharIndex - range : 0

      while (low <= high) {
        if (
          !matchesInFirstString[firstStringCharIndex] &&
          !matchesInSecondString[low] &&
          firstString.charAt(firstStringCharIndex) === secondString.charAt(low)
        ) {
          ++matchesFound
          matchesInFirstString[firstStringCharIndex] = matchesInSecondString[
            low
          ] = true
          low = high
        }
        low++
      }
    })

  if (matchesFound === 0) return 0

  let transpositionsCounterIndex = 0,
    transpositions = 0

  new Array(firstString.length)
    .fill(null)
    .forEach((_, firstStringCharIndex) => {
      if (matchesInFirstString[firstStringCharIndex]) {
        while (transpositionsCounterIndex < secondString.length) {
          if (matchesInSecondString[transpositionsCounterIndex]) {
            transpositionsCounterIndex += 1
            break
          }
          if (
            firstString.charAt(firstStringCharIndex) !==
            secondString.charAt(transpositionsCounterIndex)
          ) {
            ++transpositions
          }
          transpositionsCounterIndex++
        }
      }
    })

  let weight =
    (matchesFound / firstString.length +
      matchesFound / secondString.length +
      (matchesFound - transpositions / 2) / matchesFound) /
    3

  let lengthPrefix = 0

  const scoreScalingFactor = 0.1

  if (weight > 0.7) {
    while (
      firstString[lengthPrefix] === secondString[lengthPrefix] &&
      lengthPrefix < 4
    )
      ++lengthPrefix
    weight = weight + lengthPrefix * scoreScalingFactor * (1 - weight)
  }

  if (config?.asRatio) {
    return weight
  }

  return weight * 100
}