Promise.prototype.then()

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Parameters

onFulfilled Optional

A Function called if the Promise is fulfilled. This function has one argument, the fulfillment value. If it is not a function, it is internally replaced with an "Identity" function (it returns the received argument).

onRejected Optional

A Function called if the Promise is rejected. This function has one argument, the rejection reason. If it is not a function, it is internally replaced with a "Thrower" function (it throws an error it received as argument).

Return value

Once a Promise is fulfilled or rejected, the respective handler function (onFulfilled or onRejected) will be called asynchronously (scheduled in the current thread loop). The behavior of the handler function follows a specific set of rules. If a handler function:

• returns a value, the promise returned by then gets resolved with the returned value as its value.
• doesn't return anything, the promise returned by then gets resolved with an undefined value.
• throws an error, the promise returned by then gets rejected with the thrown error as its value.
• returns an already fulfilled promise, the promise returned by then gets fulfilled with that promise's value as its value.
• returns an already rejected promise, the promise returned by then gets rejected with that promise's value as its value.
• returns another pending promise object, the resolution/rejection of the promise returned by then will be subsequent to the resolution/rejection of the promise returned by the handler. Also, the resolved value of the promise returned by then will be the same as the resolved value of the promise returned by the handler.

When a value is returned from within a then handler, it will effectively return Promise.resolve(<value returned by whichever handler was called>).