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This article mainly shows you "what are the optional callback functions in js". The content is simple and clear. I hope it can help you solve your doubts. Let me lead you to study and learn the article "what callback functions are available in js".

1. The callback function of then is not strictly required. If you don't write, promise will also pass it with the last returned final value.

DoSomething () .then () .then (function (result) {console.log ('got a result', result);}); / / output result is: / got a result 42

2. If the current then does not transmit a callback function, the function will directly use the final value returned by the previous promise to solve the next promise.

If (! handler.onResolved) {handler.resolve (value); return;} these are all the contents of the article "what are the optional callback functions in js?" Thank you for reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!

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