Promises: resolve is not the opposite of reject

When I first started working with promises I had the overly simplistic view that passing a value into reject would mark the promise as “failed”, and passing a value into resolve would mark it as “successful”. However, the latter isn’t always true.

new Promise(function(resolve, reject) {
  resolve(something);
}).then(function() {
  console.log("Yey");
}, function() {
  console.log("Boo");
});

Even though we aren’t calling reject(), the rejection callback console.log("Boo") will be called if either:

  • something is not defined, resulting in an error being thrown, which is caught by the promise and turned into a rejection, or
  • something is a promise that rejects

So:

new Promise(function(resolve, reject) {
  resolve(Promise.reject());
}).catch(function() {
  // This is called
});

This is a good thing, as it behaves the same as Promise.resolve() and the return value from callbacks:

var promise1 = Promise.resolve(Promise.reject());
var promise2 = Promise.resolve().then(function() {
  return Promise.reject();
});
var promise3 = Promise.reject().catch(function() {
  return Promise.reject();
});

All promises above are rejected. When you resolve a value with a “then” method, you’re deferring the resolution to the eventual non-promise value.

In Practice

You can resolve a value without worrying if it’s a value, a promise, or a promise that resolves to a promise that resolves to a promise etc etc.

function apiCall(method, params) {
  return new Promise(function(resolve, reject) {
    if (!method) {
      throw TypeError("apiCall: You must provide a method");
    }

    var data = {
      jsonrpc: "2.0",
      method: method
    }

    if (params) {
      data.params = params;
    }

    resolve(postJSON('/my/api/', data));
  });
}

Now apiCall will reject if method isn’t provided, or if postJSON rejects for whatever reason. You’ve safely handed off the resolution of the promise onto postJSON.

Further reading


This content originally appeared on Jake Archibald's blog and was authored by Jake Archibald's blog

When I first started working with promises I had the overly simplistic view that passing a value into reject would mark the promise as "failed", and passing a value into resolve would mark it as "successful". However, the latter isn't always true.

new Promise(function(resolve, reject) {
  resolve(something);
}).then(function() {
  console.log("Yey");
}, function() {
  console.log("Boo");
});

Even though we aren't calling reject(), the rejection callback console.log("Boo") will be called if either:

  • something is not defined, resulting in an error being thrown, which is caught by the promise and turned into a rejection, or
  • something is a promise that rejects

So:

new Promise(function(resolve, reject) {
  resolve(Promise.reject());
}).catch(function() {
  // This is called
});

This is a good thing, as it behaves the same as Promise.resolve() and the return value from callbacks:

var promise1 = Promise.resolve(Promise.reject());
var promise2 = Promise.resolve().then(function() {
  return Promise.reject();
});
var promise3 = Promise.reject().catch(function() {
  return Promise.reject();
});

All promises above are rejected. When you resolve a value with a "then" method, you're deferring the resolution to the eventual non-promise value.

In Practice

You can resolve a value without worrying if it's a value, a promise, or a promise that resolves to a promise that resolves to a promise etc etc.

function apiCall(method, params) {
  return new Promise(function(resolve, reject) {
    if (!method) {
      throw TypeError("apiCall: You must provide a method");
    }

    var data = {
      jsonrpc: "2.0",
      method: method
    }

    if (params) {
      data.params = params;
    }

    resolve(postJSON('/my/api/', data));
  });
}

Now apiCall will reject if method isn't provided, or if postJSON rejects for whatever reason. You've safely handed off the resolution of the promise onto postJSON.

Further reading


This content originally appeared on Jake Archibald's blog and was authored by Jake Archibald's blog


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