const FILEPATH = './file/to/template.hmtl';

const fs = require('fs');

const s = fs.readFileSync(FILEPATH, 'utf-8');

const vars = Array.from(
  new Set(
    s
    .match(/{{([a-zA-Z0-9]+}})+/g)
    .map(x => x.match(/{{([a-zA-Z0-9]+)}}/)[1])
  )
);

console.log(vars.sort().join('\n'));