Add Word Strength to Duolingo

duolingo-word-strength.js

// ==UserScript==
// @name         Add Word Strength to Duolingo
// @namespace    https://gist.githubusercontent.com/nateinaction/e833488d3b67637eb5176b189c52cd0e/
// @version      0.3
// @description  Add word strength percentages to Duolingo's words page at the tap of a button.
// @author       Nate Gay
// @match        https://www.duolingo.com/words
// @run-at       document-idle
// @grant        none
// @updateUrl    https://gist.githubusercontent.com/nateinaction/e833488d3b67637eb5176b189c52cd0e/raw/
// @downloadUrl  https://gist.githubusercontent.com/nateinaction/e833488d3b67637eb5176b189c52cd0e/raw/
// @supportUrl   https://gist.githubusercontent.com/nateinaction/e833488d3b67637eb5176b189c52cd0e/
// ==/UserScript==

const addStrength = () => {
    const strong = document.querySelectorAll('span.eTLSK').length - 1;
    const good = document.querySelectorAll('span._236Ma').length - 1;
    const practice = document.querySelectorAll('span._2pMjT').length - 1;
    const overdue = document.querySelectorAll('span._2BiYf').length - 1;
    const wordGroups = [strong, good, practice, overdue];
    const all = wordGroups.reduce((a, b) => a + b);
    for (var i = 0; i < wordGroups.length; i++) {
        let div = document.getElementsByClassName('_1jxVm')[0];
        let percentStrength = (wordGroups[i]/all).toFixed(2) * 100;
        let elem = document.createElement('span');
        elem.style.marginLeft = '20px';
        elem.innerHTML = wordGroups[i] + ' (' + percentStrength + '%)  ';
        div.replaceWith(elem);
    }

    // Total words
    const overallStrength = ((strong * 4 + good * 3 + practice * 2 + overdue) / (all * 4)).toFixed(4) * 100;
    document.getElementsByClassName('_1VWgO')[0].append(' (' + overallStrength + '%)');
}

let done = false;
window.addEventListener('keydown', function() {
    if (!done) {
        addStrength();
        done = true;
    }
}, false);