What’s the difference between returning value or Promise.resolve from then()

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In simple terms, inside a then handler function:

A) When x is a value (number, string, etc):

  1. return x is equivalent to return Promise.resolve(x)
  2. throw x is equivalent to return Promise.reject(x)

B) When x is a Promise that is already settled (not pending anymore):

  1. return x is equivalent to return Promise.resolve(x), if the Promise was already resolved.
  2. return x is equivalent to return Promise.reject(x), if the Promise was already rejected.

C) When x is a Promise that is pending:

  1. return x will return a pending Promise, and it will be evaluated on the subsequent then.

Read more on this topic on the Promise.prototype.then() docs.

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