The answer
Return an object or array from each .then: .then(user => fetchPosts(user).then(posts => ({ user, posts }))).then(({ user, posts }) => ...). Or use async/await with local variables (much cleaner).
Promise Chaining in JavaScript
How do you pass multiple values through a promise chain in JavaScript?
More FAQs in Promise Chaining in JavaScript
Each .then returns a new promise. If the callback returns a value, the new promise fulfills with it. If it returns a promise, the new promise waits for it. The next .then receives the result. This lets you sequence async operations flatly.
The next .then receives undefined. The chain continues but with undefined as the value. Arrow functions with block bodies need an explicit return; expression bodies (no braces) return implicitly.
Yes. If the callback returns a promise, the next .then waits for it to settle. The next .then receives the resolved value. This is how you chain async operations that depend on each other.
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