Source: speech.js

/**
 * @file 音声合成のラッパー関数群です。
 */
/*
 * @author 市川雄二
 * @copyright 2017 ICHIKAWA, Yuji (New 3 Rs)
 * @license MIT
 */
/* global i18n */

/**
 * @param {string} text
 * @param {string} lang
 * @param {string} gender
 */
export function speak(text, lang, gender) {
    if (!SpeechSynthesisUtterance)
        return false;

    switch (lang) {
    case 'en':
        lang = 'en-us';
        break;
    case 'ja':
        lang = 'ja-jp';
        break;
    }
    const utterance = new SpeechSynthesisUtterance(text);
    if (/(iPhone|iPad|iPod)(?=.*OS [7-8])/.test(navigator.userAgent))
        utterance.rate = 0.2;
    const voices = speechSynthesis.getVoices().filter(e => e.lang.toLowerCase() === lang);
    let voice = null;
    if (voices.length > 1) {
        let names = null;
        switch (lang) {
        case 'ja-jp':
            switch (gender) {
            case 'male':
                names = ['Otoya', 'Hattori', 'Ichiro'];
                break;
            case 'female':
                names = ['O-ren（拡張）', 'O-ren', 'Kyoko', 'Haruka']; // Windows 10のAyumiの声は今ひとつ
                break;
            }
            break;
        case 'en-us':
            switch (gender) {
            case 'male':
                names = ['Alex', 'Fred'];
                break;
            case 'female':
                names = ['Samantha', 'Victoria'];
                break;
            }
            break;
        }
        if (names) {
            voice = voices.filter(v => names.some(n => v.name.indexOf(n) >= 0))[0];
        }
        if (!voice) {
            voice = voices.filter(v => v.gender && v.gender.toLowerCase() === gender)[0];
        }
    }
    utterance.voice = voice || voices[0];
    // iOS 10 Safari has a bug that utterance.voice is no effect.
    utterance.volume = parseFloat(localStorage.getItem('volume') || '1.0');
    speechSynthesis.speak(utterance);
    return true;
}

/**
 * @private
 */
function unlock() {
    window.removeEventListener('click', unlock);
    speechSynthesis.speak(new SpeechSynthesisUtterance(''));
}

/**
 * 
 * @param {string} message 
 */
export function i18nSpeak(message) {
    return speak(message, i18n.lang, 'female');
}


window.addEventListener('load', function(event) {
    if (speechSynthesis) {
        speechSynthesis.getVoices();
        if (speechSynthesis.onvoiceschanged !== undefined) {
            speechSynthesis.onvoiceschanged = function() {
                console.log('onvoiceschanged');
            }
        }
        window.addEventListener('click', unlock, false); // for iOS
    }
});