index.js

var through = require("through2");
var log = require("fancy-log");
var PluginError = require("plugin-error");
var colors = require("ansi-colors");

const PLUGIN_NAME = "gulp-json-transform";

function jsonPromiseParse(rawStr) {
  return new Promise(function (resolve, reject) {
    var json;
    try {
      json = JSON.parse(rawStr);
    } catch (e) {
      return reject(new Error("Invalid JSON: " + e.message));
    }
    resolve(json);
  });
}

module.exports = function (transformFn, jsonSpace) {
  if (!transformFn) {
    throw new PluginError(PLUGIN_NAME, "Missing transform function!");
  }

  return through.obj(function (file, enc, cb) {
    var self = this;

    if (file.isStream()) {
      return self.emit(
        "error",
        new PluginError(PLUGIN_NAME, "Streaming not supported")
      );
    }

    if (file.isBuffer()) {
      var fileContent = file.contents.toString(enc);

      jsonPromiseParse(fileContent)
        .then(function (data) {
          return transformFn(data, {
            path: file.path,
            relative: file.relative,
            base: file.base,
          });
        })
        .then(function (output) {
          var isString = typeof output === "string";
          file.contents = Buffer.from(
            isString ? output : JSON.stringify(output, null, jsonSpace)
          );
          self.push(file);
          cb();
        })
        .catch(function (e) {
          log(PLUGIN_NAME + ":", colors.red(e.message));
          self.emit("error", new PluginError(PLUGIN_NAME, e));
          self.emit("end");
        });
    }
  });
};