text: let x = 0;
  // false, 0, "", undefined,null,NaN
  
  if (x) {
      console.log("this is  truthy value");
  } else {
      console.log("this is falsey value");
  }
  
  let frog1 = "Kate";
  let frog2 = "Peter";
  let frog3 = "Jade";
  
  let frogs = ["John", "Sandy", "Alex", "Jim"];
  
  //index , length
  console.log(frogs[0]);
  console.log(frogs.length);
  // push , pop, shift, unshift  4 methods
  
  console.log(frogs.pop()); // 後減 1
  console.log(frogs.push("Alice")); //後加 1
  console.log(frogs.shift()); // 前 減 1
  console.log(frogs.unshift("Adele")); //前 加 1
  
  console.log(frogs);
  
  let xxyy = frogs.unshift("Peter");
  console.log(frogs);
  console.log(xxyy); //會回傳frogs 的長度
  
  //Celsius to Fahrenheit
  //多一行 ，可是可以 再運算  ，複 雜 時好用
  function convert(oc) {
      return oc * 1.8 + 32;
  }
  let yt = convert(0);
  console.log(yt);
  
  //少 一行  函數等於 輸出
  function convert1(oc1) {
      of1 = oc1 * 1.8 + 32;
      return console.log(of1);
  }
  convert1(0);
  