JavaScript中Promise不同调用方式的执行差异及异常问题咨询

Great question! Let's break down exactly what's happening in both of your Promise scenarios—this gets to the core of how Promises handle callbacks, state changes, and error propagation.

First Scenario: p.then(doSomething1); p.then(doSomething2); p.catch(doSomething3) #

When you attach multiple .then() calls directly to the original Promise p, here's the breakdown:

• Each .then() registers an independent callback that listens for p's state change.
• Assuming somePromise resolves successfully (since you see the sequence 1->2), all callbacks attached directly to p will execute in the order they were registered.
• First doSomething1 runs (outputting 1), then doSomething2 runs (outputting 2).
• The .catch(doSomething3) only triggers if p itself is rejected. Since p resolved without errors, this callback never executes.

That's why you see the sequence 1->2.

Second Scenario: p.then(doSomething1).then(() => { throw some error }); p.then(doSomething2); p.catch(doSomething3) #

This scenario hinges on a key Promise behavior: every .then() or .catch() call returns a brand new Promise instance. Let's break it down step by step:

1. p.then(doSomething1) creates a new Promise (let's call it p1). Since doSomething1 runs successfully (as in the first scenario), p1 resolves.
2. p1.then(() => { throw new Error() }) creates another new Promise (p2). When the callback throws an error, p2 transitions to a rejected state.
3. Critical note: p2 is a completely separate Promise from the original p. The error in p2 does not affect p's state—p remains resolved!

Why you see 1->3 and an unhandled rejection warning #

Let's unpack these two outcomes:

• Unhandled rejection warning: The rejected Promise p2 has no .catch() callback attached to it. JavaScript engines flag this because it's an uncaught error in a Promise chain that no code is handling.
• Why doSomething3 runs: If your code actually has the .catch(doSomething3) attached to the chained Promise (instead of the original p, like p.then(doSomething1).then(() => { throw error }).catch(doSomething3)), then doSomething3 would run to handle p2's error. If it's attached to p, it shouldn't run—unless p itself was rejected elsewhere (which contradicts your first scenario's success).

The core lessons here are:

• Chained Promises are independent of the original Promise. Errors in a chain only affect the new Promises created by .then()/.catch() calls.
• Callbacks attached directly to the original Promise only react to the original Promise's state changes, not changes in chained Promises.

内容的提问来源于stack exchange，提问作者moon.pike