React状态更新合并疑问：为何setState更新comments不影响posts？

Great question—this gets to a key behavior of React's state management that trips up a lot of developers at first. Let's break it down step by step:

1. React Uses Shallow Merging for State Updates #

When you call setState(), React doesn't replace your entire state object with the one you pass in. Instead, it does a shallow merge:

• It takes the keys from the object you provide to setState()
• It updates those keys in the existing state with your new values
• Any keys you don't mention in the update object stay exactly as they were in the previous state

2. Example to Illustrate #

Let's say your initial state looks like this:

this.state = {
  posts: ['Post 1', 'Post 2'],
  comments: ['Comment A']
};

When you run this.setState({comments: ['Comment A', 'Comment B']}), here's what happens:

• React sees you're updating the comments key, so it replaces the old comments array with your new one
• Since you didn't include posts in the update object, React leaves the existing posts array untouched

The resulting state becomes:

{
  posts: ['Post 1', 'Post 2'], // Unchanged
  comments: ['Comment A', 'Comment B'] // Updated
}

3. Why comments Gets Fully Replaced #

This part is important: React doesn't do deep merging of nested values. If you pass a new value for a top-level key (like comments), it replaces the entire value for that key—even if it's an array or object.

If you wanted to add a comment instead of replacing the whole array, you'd need to create a new array that includes the old comments plus the new one, like:

this.setState({
  comments: [...this.state.comments, 'New Comment']
});

Recap #

To sum it all up:

• setState() merges your update object with the current state (shallowly)
• Keys you include in the update object get their values replaced
• Keys you don't include stay exactly as they were
• That's why this.state.posts remains unchanged, while this.state.comments is fully replaced with the new value you passed in

内容的提问来源于stack exchange，提问作者ZIYIM