JavaScript Implementation of the Flesch Reading-Ease algorithm to determine how difficult a passage in English is to understand.

FleschReadingEase.js

/**
 * Copyright (c) 2021
 *
 * JavaScript Implementation of the Flesch Reading-Ease algorithm to determine how difficult a passage in English is to understand.
 * Reference: https://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readability_tests
 * @summary Flesch Reading-Ease algorithm to determine how difficult a passage in English is to understand.
 * @author Boulama K. <boulama@otimbi.com>
 */


/*
Function to calculate the number of syllables in a word.
*/

function countSyllables(word) {
  word = word.toLowerCase();                                     //word.downcase!
  if(word.length <= 3) { return 1; }                             //return 1 if word.length <= 3
    word = word.replace(/(?:[^laeiouy]es|ed|[^laeiouy]e)$/, '');   //word.sub!(/(?:[^laeiouy]es|ed|[^laeiouy]e)$/, '')
    word = word.replace(/^y/, '');                                 //word.sub!(/^y/, '')
    return word.match(/[aeiouy]{1,2}/g).length;                    //word.scan(/[aeiouy]{1,2}/).size
}

/*
Function to calculate the number of syllables in a piece of text.
*/

function countAllSylables(text) {
	let all_words = getAllWords(text);
  let total_words = all_words.length;
  
  var total_syllables = 0;
  
  
  for(i=0;i<total_words;i++) {
  	total_syllables += countSyllables(all_words[i]);
  }
	return total_syllables;
}


/*
Function to get the words present in a piece of text.
*/

function getAllWords(text) {
	return text.split(' ');
}


/*
Function to calculate the number of words present in a piece of text.
*/

function countWords(text) {
	return getAllWords(text).length;
}


/*
Function to calculate the number of sentences.
*/

function countSentences(text) {
	let sentences = text.split('.');
	let total_sentences = sentences.length;
  var total_actual_sentences = 0;
  
  for(i=0;i<total_sentences;i++) {
  	var current_string = sentences[i];
    current_string = current_string.replace(/\s/g, "");
    
    /*
     I will determine here the minimum length of a sentence to be 5 characters to be considered.
    */
  	if(current_string.length > 5) {
  		total_actual_sentences++
    }
  }
  
  return total_actual_sentences;
}



/*
Calculate the readability score of a piece of text using the Flesch–Kincaid readability algorithm
Reference: https://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readability_tests
*/


function readability(text = null) {
    let total_words = countWords(text);
    let total_sentences = countSentences(text);
    let total_syllables = countAllSylables(text);
    let school_level;
    let notes;
    var FK = 206.835 - (1.015 * (total_words/total_sentences)) - (84.6 * (total_syllables/total_words));
    
    console.log(FK);

    if(FK >= 90 && FK <= 100) {
        school_level = "5th grade"
        notes = "Very easy to read. Easily understood by an average 11-year-old student."
    }

    if(FK >= 80 && FK < 90) {
        school_level = "6th grade"
        notes = "Easy to read. Conversational English for consumers."
    }

    if(FK >= 70 && FK < 80) {
        school_level = "7th grade"
        notes = "Fairly easy to read."
    }

    if(FK >= 60 && FK < 70) {
        school_level = "8th & 9th grade"
        notes = "Plain English. Easily understood by 13- to 15-year-old students."
    }

    if(FK >= 50 && FK < 60) {
        school_level = "10th to 12th grade"
        notes = "Fairly difficult to read."
    }

    if(FK >= 30 && FK < 50) {
        school_level = "College"
        notes = "Difficult to read."
    }

    if(FK >= 10 && FK < 30) {
        school_level = "College graduate"
        notes = "Very difficult to read. Best understood by university graduates."
    }

    if(FK >= 0 && FK < 10) {
        school_level = "Professional"
        notes = "Extremely difficult to read. Best understood by university graduates."
    }

    return [school_level, notes];
}