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Napoleon Bonaparte
Napoleon Bonaparte

Reputation: 114

Passing a primitive to a then handler

This is an interesting observation that I have made when experimenting with promises in JS. Consider this code:

Promise.resolve(1).then(2).then(console.log);

This prints out 1 as its output. Why didn't the native-implemented promise API throw an exception? What is the benefit of "skipping" the non-function argument when it is being ignored anyway? I woud really apreciate an answer

This post is found in MDN.

Upvotes: 2

Views: 80

Answers (1)

Nicholas Tower
Nicholas Tower

Reputation: 84957

The way modern promises work is defined by the ECMAScript spec. But promises already existed before they were added as a core part of the language, so for backwards compatibility they were designed to match the behavior of community-built promises. The spec that was used by the community is called Promises/A+, and the relevant part is this:

2.2 The then method

A promise’s then method accepts two arguments:
promise.then(onFulfilled, onRejected)

2.2.1 Both onFulfilled and onRejected are optional arguments:
2.2.1.1 If onFulfilled is not a function, it must be ignored.
2.2.1.2 If onRejected is not a function, it must be ignored.

ECMAScript has inherited that behavior in its spec. That spec is much harder to read, but i think it's this part from PerformPromiseThen

  1. If IsCallable(onFulfilled) is false, then
    a. Let onFulfilledJobCallback be empty.

As for why they chose to design them this way, I don't know.

Upvotes: 4

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