Ghost/core/frontend/services/helpers/handlebars.js

const Promise = require('bluebird');
const errors = require('@tryghost/errors');
const logging = require('@tryghost/logging');

const {hbs} = require('../handlebars');

// Register an async handlebars helper for a given handlebars instance
function asyncHelperWrapper(hbsInstance, name, fn) {
    hbsInstance.registerAsyncHelper(name, function returnAsync(context, options, cb) {
        // Handle the case where we only get context and cb
        if (!cb) {
            cb = options;
            options = undefined;
        }

        // Wrap the function passed in with a Promise.resolve so it can return either a promise or a value
        Promise.resolve(fn.call(this, context, options)).then(function asyncHelperSuccess(result) {
            cb(result);
        }).catch(function asyncHelperError(err) {
            const wrappedErr = errors.utils.isGhostError(err) ? err : new errors.IncorrectUsageError({
                err: err,
                context: 'registerAsyncThemeHelper: ' + name,
                errorDetails: {
                    originalError: err
                }
            });

            const result = process.env.NODE_ENV === 'development' ? wrappedErr : '';

            logging.error(wrappedErr);

            cb(new hbsInstance.SafeString(result));
        });
    });
}

// Register a handlebars helper for themes
module.exports.registerThemeHelper = function registerThemeHelper(name, fn) {
    hbs.registerHelper(name, fn);
};

// Register an async handlebars helper for themes
module.exports.registerAsyncThemeHelper = function registerAsyncThemeHelper(name, fn) {
    asyncHelperWrapper(hbs, name, fn);
};