Recursive Generators in JavaScript

The problem was not with the recursion. Your function did call itself recursively, it just didn’t yield values outside. When you call helper(), you get an iterator as a return value, but you wanted the iterated values of that iterator to be yielded. If you want to yield recursively, you need to yield *. Try like this:

  * inOrderTraversal() {
    function* helper(node) {
      if (node.left !== null) {
        // this line is executed, but helper is not being called
        yield * helper(node.left); 
      yield node.value;
      if (node.right !== null) {
        yield * helper(node.right);

    for (let i of helper(this.root)) {
      yield i;

And while you’re at it, you can replace the for loop with:

yield * helper(this.root)

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