for-inとfor-of

for in - {}オブジェクト

 var object = { x: 100, y: 200, z: 300 };
 for (var key in object) {
     console.log(key, object[key]);
 }
 
 object.x = undefined;
 console.log(Object.keys(object).length);  # 3
 delete object.x;
 console.log(Object.keys(object).length);  # 2

• https://developer.mozilla.org/ja/docs/Web/JavaScript/Reference/Statements/for...in

for of - 配列などiterableなオブジェクト

 var array = [100, 200, 300];
 for (var v of array) {
     console.log(v);
 }

• for-inの{}はiterableではないのでfor-ofは使えない
• https://developer.mozilla.org/ja/docs/Web/JavaScript/Reference/Statements/for...of