yield*

file_tree.js

const fs = require('fs'); // the node file system package
const path = require('path');

// Usage: 
//    node file_tree.js name_of_folder_to_get_files_from 
//    node file_tree.js

// Either a passed-in-folder name (process.argv[2])
// or default to the current working directory
const folder = process.argv[2] || process.cwd();

// (Function definition is below)
const allNestedFiles = walkFileTree(folder);

// We can use `for ... of` here because of the generator :D
for (const file of allNestedFiles) {
  console.log(file);
}

// -----------------------------------------------------------------------------
function* walkFileTree(startingFolder = '.') {
  const readOptions = { withFileTypes: true };
  const folderContents = fs.readdirSync(startingFolder, readOptions);

  for (const currentItem of folderContents) {
    const fullyQualifiedName = path.join(startingFolder, currentItem.name);
    if (currentItem.isDirectory()) {
      // *this* is the super nifty party -- it will `yield`, in turn, EVERYTHING
      //    that `walkFileTree` itself then yields -- this is how we go deeper
      yield* walkFileTree(fullyQualifiedName);
    } else {
      yield fullyQualifiedName;
    }
  }
}

generator_ception.js

function* generateLetters() {
  const letters = 'abc';
  for (const letter of letters) {
    yield letter;
  }
}

function* generateNumbers() {
  // a very clunky range
  // the numbers from 0 to 2
  const numbers = new Array(3).fill(null).map((_, index) => index);
  for (const number of numbers) {
    yield number;
  }
}

function* interruptingLetters() {
  const numbers = new Array(3).fill(null).map((_, index) => index);
  for (const number of numbers) {
    yield number;
    if (number % 2 === 1) {
      // if our number is odd
      yield* generateLetters();
    }
  }
}

const lettersJustGenerated = [...generateLetters()];
console.log(lettersJustGenerated); // [ 'a', 'b', 'c' ]

const numbersJustGenerated = [...generateNumbers()];
console.log(numbersJustGenerated); // [ 0, 1, 2 ]

const intermixed = [...interruptingLetters()];
console.log(intermixed); // [ 0, 1, 'a', 'b', 'c', 2 ]


// yield 0
// yield 1
    // yield* ...
    // yield 'a', yield 'b', yield 'c'
// yield 2