index.js

#! /usr/bin/env node
var pandoc=require('pandoc-filter');
var _ = require('lodash');
var tmp = require('tmp');
var fs = require('fs');
var path = require('path');
var exec = require('child_process').execSync;

var prefix="diagram";
var outdir= "img";
var mermaid_opts = "-v -o " + outdir;
var cmd = "mermaid " + mermaid_opts;
var counter = 0;
function mermaid(type, value, format, meta) {
    if (type != "CodeBlock") return null;
    var attrs = value[0],
        content = value[1];
    var classes = attrs[1];
    var options = {width: '500', format: 'png'};
    if (classes.indexOf('mermaid') < 0) return null;

    counter++;
    attrs[2].map(item => {
        if (item.length  === 1) options[item[0]] = true;
        else options[item[0]] = item[1];
    });
    //console.log(options);
    if (!_.contains('mermaid', classes)) return null;
    counter++;
    //console.log(content);
    var tmpfileObj = tmp.fileSync();
    //console.log(tmpfile);
    fs.writeFileSync(tmpfileObj.name, content);
    var oldPath = path.join(outdir, path.basename(tmpfileObj.name) + `.${options.format}`);
    var newPath = path.join(outdir, `${prefix}-${counter}.${options.format}`);
    exec(`${cmd} -w ${options.width} ${options.format==='png' ? "-p": "-s"}  ${tmpfileObj.name}`);
    //console.log(oldPath, newPath);
    fs.renameSync(oldPath, newPath);
    return pandoc.Para(
                [
                    pandoc.Image(
                        ['', [], []],
                        [],
                        [newPath, ""]
                    )
                ]);
}

pandoc.toJSONFilter(mermaid);