根据对象值更改对象值

Question

以下是我的console.log(myJson)

的结果

[{"placeholdId":1,"itemId":null,"type":null},
{"placeholdId":2,"itemId":null,"type":null},
{"placeholdId":3,"itemId":null,"type":null},
{"placeholdId":4,"itemId":null,"type":null},
{"placeholdId":5,"itemId":5,"type":"food"},
{"placeholdId":6,"itemId":null,"type":"null"}]

如何根据placeholdId设置对象值的值？例如第5项，我想更改itemId并输入null。

我被困在这里..

var obj = $.parseJSON(json);
console.log(obj[4]);