trac.js

const htmlToText = html => {
    const node = document.createElement('div');
    node.innerHTML = html;

    node.querySelectorAll('b, strong').forEach(
        fontNode => fontNode.innerHTML = `**${fontNode.innerHTML}**`
    );

    node.querySelectorAll('i, em').forEach(
        fontNode => fontNode.innerHTML = `//${fontNode.innerHTML}//`
    );

    node.querySelectorAll('code').forEach(
        codeNode => codeNode.innerHTML = `\`${codeNode.innerHTML}\``
    );

    node.querySelectorAll('a').forEach(
        linkNode => linkNode.outerHTML = `[${linkNode.getAttribute('href')} ${linkNode.innerText}]`
    );

    return node.innerText;
}

const blockToTrac = (state, block) => {
    /**
     * Convert a number to Roman Numerals.
     *
     * @cite https://stackoverflow.com/a/9083076/486538
     */
    const romanize = num => {
        const digits = String(+num).split('');
        const key = ["", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM", "", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC", "", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"];
        let roman = "";
        let i = 3;
        while (i--) {
            roman = (key[+digits.pop() + (i * 10)] || "") + roman;
        }
        return Array(+digits.join("") + 1).join("M") + roman;
    };

    switch (block.name) {
        case 'core/quote':
            const content = blocksToTrac(state, block.innerBlocks);
            return content.split(/\n/g).map(l => `> ${l}`).join('\n') + '\n\n';

        case 'core/code':
            const code = htmlToText(block.attributes.content);
            const languageSpec = code.startsWith('<?php') ? `#!php` : '';
            return `{{{${languageSpec}\n${code}\n}}}\n\n`;

        case 'core/heading':
            return '='.repeat(block.attributes.level) + ' ' + htmlToText(block.attributes.content) + '\n\n';

        case 'core/list':
            state.indent++;
            state.listStyle.push({
                style: block.attributes.ordered ? (block.attributes.type || 'decimal') : '-',
                count: block.attributes.start || 1
            });
            const list = blocksToTrac(state, block.innerBlocks);
            state.listStyle.pop();
            state.indent--;
            return `${list}\n\n`;

        case 'core/list-item':
            if (0 === state.listStyle.length) {
                return '';
            }

            const item = state.listStyle[state.listStyle.length - 1];
            const bullet = (() => {
                switch (item.style) {
                    case '-':
                        return '-';

                    case 'decimal':
                        return `${item.count.toString()}.`;

                    case 'upper-alpha': {
                        let count = item.count;
                        let bullet = '';
                        while (count >= 1) {
                            bullet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'[(count - 1) % 26] + bullet;
                            count /= 26;
                        }
                        return `${bullet}.`;
                    }

                    case 'lower-alpha': {
                        let count = item.count;
                        let bullet = '';
                        while (count >= 1) {
                            bullet = 'abcdefghijklmnopqrstuvwxyz'[(count - 1) % 26] + bullet;
                            count /= 26;
                        }
                        return `${bullet}.`;
                    }

                    case 'upper-roman':
                        return romanize(item.count) + '.';

                    case 'lower-roman':
                        return romanize(item.count).toLowerCase();

                    default:
                        return `${item.count.toString()}.`;
                }
            })();

            item.count++;

            return `${'\t'.repeat(state.indent)}${bullet} ${htmlToText(block.attributes.content)}\n`;

        case 'core/paragraph':
            if ('undefined' === typeof window.lastContent) {
                window.lastContent = block.attributes.content;
            }
            console.log(window.lastContent);
            return htmlToText(block.attributes.content) + '\n\n';

        case 'core/separator':
            return '\n----\n\n';

        default:
            return '';
    }
}

const blocksToTrac = (state, blocks) => {
    return blocks.map(block => blockToTrac(state, block)).join('').replace(/^[\n\r]+|[\n\r]+$/g, '');
}

export const blocks2trac = (blocks) => {
    const state = {
        indent: 0,
        listStyle: [],
    };

    return blocksToTrac(state, blocks || []);
}