index.js

function CachingWriter(inputTrees, options) {
  if (Array.isArray(inputTrees)) {
    if (this.enforceSingleInputTree) { // <---- in ES6, `this` is not defined before super
      throw new Error('You passed an array of input trees, but only a single tree is allowed.');
    }
    inputTrees = inputTrees;
  } else {
    inputTrees = [inputTrees];
  }
  Plugin.call(this, inputTrees);
  
  // ...
}