Anti-if Stack Implementation

unconditional-stack.js

// idea from: https://www.youtube.com/watch?v=APUCMSPiNh4

class IllegalStateError extends Error {
  constructor(...args) {
    super(...args);
  }
}

class Empty {
  size() {
    return 0;
  }

  top() {
    throw new IllegalStateError('The stack is empty.');
  }
  
  pop() {
    throw new IllegalStateError('The stack is empty.');
  }

  push(newTop) {
    return new NonEmpty(newTop, this);
  }
}

class NonEmpty {
  constructor(newTop, previous) {
    this.top = newTop;
    this.tail = previous;
  }

  size() {
    return 1 + this.tail.size();
  }

  top() {
    return this.top;
  }

  pop() {
    return this.tail;
  }

  push(newTop) {
    return new NonEmpty(newTop, this);
  }
}

const stack = new Empty().push('foo').push('bar');
console.log(stack.tail);