node.js script that converts all `.fbx` assets within a directory to the `.gltf` file format. Requires FBX2glTF (https://github.com/facebookincubator/FBX2glTF) and fs-extra (https://github.com/jprichardson/node-fs-extra). Assumes usage of a module bundler that allows for importing files for use in JS (such as Webpack or Rollup).

convert-fbx-gltf.js

/**
 * Converts all .fbx files in the provided folder to .gltf files.
 * @param {String} src
 * @param {String} dest
 */

const spawn = require('child_process').spawn;
const fs = require('fs-extra');
const path = require('path');

const ART_SRC = process.argv[2] || './art';
const ART_DEST = process.argv[3] || './src/assets/art';

/**
 * Writes a JS module importing the gltf model and all of its dependencies.
 * @param {string} target The path to the gltf output.
 * @return {Promise} A promise to write the module out.
 */
function writeModelModule(target) {
  const output = `${target}_out`;
  const filename = path.basename(target);
  const importDir = `${filename}_out`;
  return fs.readdir(output)
    // Filter out the build files
    .then(files => files.filter(file => path.extname(file).toLowerCase() !== '.gltf'))
    // Create imports for each asset
    .then(assets => assets.map(asset => `import './${importDir}/${asset}';`))
    // Create the module content
    .then(imports =>
      `import ${filename} from './${importDir}/${filename}.gltf';\n` +
      imports.join('\n') +
      `\nexport default ${filename};\n`
    )
    .then(moduleContent => fs.outputFile(`${target}.js`, moduleContent));
}

/**
 * Converts an fbx file at the given path to a gltf file.
 * @param {string} file The path to the fbx file.
 * @return {Promise<boolean>} A boolean representing the result.
 */
function convertFile(file) {
  return new Promise((resolve, reject) => {
    const target = path.join(ART_DEST, path.basename(file, '.fbx'));
    try {
      const convert = spawn('./node_modules/fbx2gltf/bin/Darwin/FBX2glTF', [
        '-i',
        file,
        '-o',
        target,
        '-d', // Enable draco compression
        '-e', // Inline buffers within non-binary gltf
        // '-v', // Verbose output
        '--khr-materials-unlit', // Unlit shader
      ]);

      convert.stdout.on('data', data => process.stdout.write(data));
      convert.stderr.on('data', data => process.stderr.write(data));
      convert.on('close', code => {
        process.stdout.write('Writing helper model JS module\n\n');
        writeModelModule(target)
          .then(() => resolve({ target, status: true }));
      });
    } catch (e) {
      process.stderr.write(`There was an error: ${e}`);
      resolve({ target, status: false });
    }
  });
}

/**
 * Recurse through a directory to try to find fbx files.
 * @param {string} dir The directory.
 * @return {Promise<string[]>} A list of fbx paths.
 */
async function recurseDirectory(dir) {
  let fbx = [];

  try {
    const subdirs = [];
    const files = await fs.readdir(dir);

    files.map(file => {
      const ext = path.extname(file);
      if (ext.toLowerCase() === '.fbx') {
        fbx.push(path.join(dir, file));
      } else if (ext.toLowerCase() === '' && file.indexOf('.') < 0) {
        subdirs.push(path.join(dir, file));
      }
    });

    for (let i = 0; i < subdirs.length; i++) {
      const subfiles = await recurseDirectory(subdirs[i]);
      fbx = fbx.concat(subfiles);
    }
  } catch (e) {
    console.error(e);
  }

  return fbx;
}

// Open up the source directory
fs.pathExists(ART_SRC, async function(err, exists) {
  if (err || !exists) {
    return;
  }

  // Find all of the files in the directory & its children
  process.stdout.write(`Reading directory ${ART_SRC}...\n`);
  const models = await recurseDirectory(ART_SRC);
  const numModels = models.length;
  process.stdout.write(`Found ${numModels}  fbx files.\n`);
  
  // Clean the target directory 
  process.stdout.write(`Cleaning target directory: ${ART_DEST}\n`);
  await fs.emptyDir(ART_DEST);
  
  // Iterate through all models and convert them to glTF
  Promise.all(
    models.map((model, i) => {
      process.stdout.write(`Converting model ${(i + 1)} of ${numModels}\n`);
      return convertFile(model);
    })
  ).then(results => {
    // Write the results of all of the files
    process.stdout.write('Finished converting files:\n\n');
    results.map((val, index) => {
      process.stdout.write(`${models[index]}\n`);
      if (val.status) {
        process.stdout.write(`Success ->\t${val.target}\n`);
      } else {
        process.stdout.write('ERROR!');
      }
    });
  }).catch(e => {
    process.stderr.write(`There was an error converting the files :(`);
  });
});