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Promise flattening is an informal name for JavaScript’s promise-resolution behavior: when a promise is resolved with another promise or thenable, it adopts that object’s eventual outcome instead of fulfilling with the object itself. This lets asynchronous steps compose into a single promise chain.
What promise flattening means
A thenable is an object with a then method. Native promises are thenables, and JavaScript’s promise resolution interoperates with other thenables by adopting the outcome they provide. MDN’s Promise reference describes this resolution behavior.
For example, Promise.resolve(value) fulfills with an ordinary, non-thenable value. If given a promise or thenable, it follows that object instead. If the thenable fulfills with another thenable, resolution adopts that one too. The eventual fulfillment value is therefore not wrapped in nested promise layers.
const nested = {
then(onFulfilled) {
onFulfilled({
then(onFulfilledAgain) {
onFulfilledAgain(42);
},
});
},
};
Promise.resolve(nested).then((value) => {
console.log(value); // 42
});The nested object is not a native Promise, but its then method makes it a thenable. Resolution follows its fulfillment callback and then the second thenable’s callback, so the handler receives 42.
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Resolved does not necessarily mean fulfilled
In precise Promise terminology, resolved and fulfilled are not interchangeable. A promise is resolved when its outcome has been locked to another promise or thenable. It may remain pending while that outcome is pending, and it may ultimately reject. Fulfillment is the successful settled state. See MDN’s explanation of promise states and fates.
So resolve(innerPromise) does not make the outer promise fulfill with innerPromise as a regular value. It makes the outer promise follow the inner one. If the inner promise fulfills, the outer one fulfills with its value; if it rejects, the outer one rejects.
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How flattening works with Promise.resolve()
Promise.resolve(value) is a convenient way to observe promise resolution:
- For a non-thenable value, it returns a promise fulfilled with that value.
- For a thenable, it adopts the outcome produced through that thenable’s
thenmethod, including further thenables supplied during resolution. - If the input is a Promise whose constructor is the current
Promise, it returns that same instance. For other inputs, resolution behaves essentially like creating a new promise and resolving it with the input.
These rules are documented in MDN’s Promise.resolve() reference. The same-constructor case matters when reasoning about identity: calling Promise.resolve(existingPromise) does not necessarily create a distinct promise.
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Do not assume that every method named resolve behaves identically. A custom promise-like constructor that borrows Promise.resolve may not assimilate nested thenables if its own resolve implementation lacks that behavior.
How flattening works in a .then() chain
Every call to .then() returns a new promise. That new promise follows the value returned by the handler: an ordinary value fulfills it, while a returned promise or thenable is adopted. This is how a chain represents successive asynchronous operations without producing a promise nested inside another promise.
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fetchData()
.then((data) => saveData(data))
.then((saved) => showResult(saved));If saveData(data) returns a promise, the second handler waits for its outcome and receives its fulfillment value. Microsoft Learn’s then() reference describes the returned promise relationship.
If a handler throws, or returns a rejected promise, the next promise rejects. That rejection continues down the chain unless a later handler handles it. Returning asynchronous work from a handler keeps it connected to the chain; starting it without returning it leaves the chain unable to wait for that work or propagate its rejection.
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Thenables and edge cases
Thenables can be useful for interoperability, but they are arbitrary objects: their authors define what their then methods do. Promise resolution uses those methods to adopt an outcome, rather than treating every thenable as a native Promise.
- A thenable resolves to itself: this creates recursive assimilation and can lead to infinite recursion, as MDN warns in its Promise.resolve() documentation.
- A custom constructor has its own resolution logic: whether it assimilates nested thenables depends on that implementation; the method name alone is not a guarantee.
Flattening describes how outcomes are adopted; it is not a claim about exactly when handlers run. Handler scheduling is a separate aspect of promise behavior.
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