Promiseあれこれ

async-await等価.js

(function () {
  const test = async () => {
    await Promise.resolve();
    return Promise.reject();
  };
  test().then(() => console.log("then"), () => console.log("reject"));
})();

(function () {
  const test = () => {
    return Promise.resolve().then(() => {
      return Promise.reject();
    });
  };
  test().then(() => console.log("then"), () => console.log("reject"));
})();

async1.js

//async宣言しただけでtestはPromiseを返すようになる
(function () {
  const test = async () => {};
  test().then(() => { console.log("then") });
})();

async2.js

//rejectするPromiseを返すかthrowすれば、rejectハンドリングできる

(function () {
  const test = async () => {
    return Promise.reject();
  };
  test().then(() => console.log("then"), () => console.log("reject"));
})();


(function () {
  const test = async () => {
    throw undefined;
  };
  test().then(() => console.log("then"), () => console.log("reject"));
})();

async等価.js

(function () {
  const test = async () => Promise.reject()
  test().then(() => console.log("then"), () => console.log("reject"));
})();

(function () {
  const test = () => {
    return Promise.reject();
  };
  test().then(() => console.log("then"), () => console.log("reject"));
})();

直感と反する1.js

Promise.resolve()
  .then(() => Promise.reject("FOO"))
  .then(() => console.log(1)) // <- skip
  .then(() => console.log(2)) // <- skip
  .then(() => console.log(3)) // <- skip
  .then(
    undefined,
    function(e){ // <- handle reject("FOO")
      console.log(e);
    }
);

等価1.js

Promise.resolve()
  .then(() => Promise.reject(new Error("FOO")))
  .catch(result => {
    console.log(result.constructor.name);
  });

Promise.resolve()
  .then(() => {
    throw new Error("BAR");
  })
  .catch(result => {
    console.log(result.constructor.name);
  });

等価2.js

Promise.resolve()
  .then(() => Promise.reject("FOO"))
  .catch(result => {
    console.log(result.constructor.name);
  });

Promise.resolve()
  .then(() => {
    throw "BAR";
  })
  .catch(result => {
    console.log(result.constructor.name);
  });

等価3.js

Promise.resolve()
  .then(() => Promise.reject("FOO"))
  .then(
    undefined,
    function(e){
      console.log(e);
    }
);

Promise.resolve()
  .then(() => {
    throw "BAR";
  })
  .catch(result => {
    console.log(result.constructor.name);
  });