第一题

class Stack {
  constructor() {
    this.stack = [];
  }
  push(item) {
    stack.push(item);
  }
  pop() {
    return stack.pop();
  }
  peek() {
    return stack[stack.length - 1];
  }
}

第二题
错误，错误提示是

Uncaught ReferenceError: Cannot access 'num' before initialization

老师，我看不出来哪里错误

function convertToBinary(num) {
  const stack = [];
  while (num !== 0) {
    let num = num / 2;
    let remainder = num % 2;
    stack.push(remainder);
  }
  return stack.join("").reverse();
}