Promise & Error All In One

Promise & Error All In One

promise errors

1. custom error & reject

const promise = new Promise((resolve, reject) => {
  // 使用 Error 对象
  reject(new Error("❌!"));
});

promise.catch(reason => {
  console.log(`reason =`, reason, typeof reason);
  console.log(`reason.message =`, reason.message); // "❌!"
  throw new Error("Oops!");
}).catch(reason => {
  console.log(`reason =`, reason, typeof reason);
  console.log(`reason.message =`, reason.message);  // "Oops!"
});

const promise = new Promise((resolve, reject) => {
  // 手动构造，模仿 Error 对象
  reject({
    message: "❌!",
  });
});

promise.catch(reason => {
  console.log(`reason =`, reason, typeof reason);
  console.log(`reason.message =`, reason.message); // "Uh oh!"
  throw new Error("Oops!");
}).catch(reason => {
  console.log(`reason =`, reason, typeof reason);
  console.log(`reason.message =`, reason.message);  // "Oops!"
});

const promise = new Promise((resolve, reject) => {
  // 字符串 error
  reject("❌!");
});

promise.catch(reason => {
  console.log(`reason =`, reason, typeof reason);
  console.log(`reason.message =`, reason.message); // undefined
  throw new Error("Oops!");
}).catch(reason => {
  console.log(`reason =`, reason, typeof reason);
  console.log(`reason.message =`, reason.message);  // "Oops!"
});

2. throw new Error & Promise

const promise = new Promise((resolve, reject) => {
  throw new Error("Uh oh!");
});

promise.catch(reason => {
  console.log(`reason =`, reason, typeof reason);
  console.log(`reason.message =`, reason.message); // "Uh oh!"
  throw new Error("Oops!");
}).catch(reason => {
  console.log(`reason =`, reason, typeof reason);
  console.log(`reason.message =`, reason.message);  // "Oops!"
});

const promise = new Promise((resolve, reject) => {
  throw new Error("Uh oh!");
});

promise.catch(reason => {
  console.error(`reason =`, reason, typeof reason);
  console.log(`reason.message =`, reason.message); // "Uh oh!"
  throw new Error("Oops!");
}).catch(reason => {
  console.error(`reason =`, reason, typeof reason);
  console.log(`reason.message =`, reason.message);  // "Oops!"
});

try...catch & Error object

try {
  throw new Error("Whoops!");
} catch (err) {
  console.log(`err =`, typeof err);
  console.log(`typeof err =`, typeof err);
  console.log(JSON.stringify(err, null, 4));
  console.error(`err.name = ${err.name}, err.message = ${err.message}`);
}

let err = throw new Error("Whoops!");
//  Uncaught SyntaxError: Unexpected token 'throw'
err;
// Uncaught ReferenceError: err is not defined

throw new Error("Whoops!");
// Uncaught Error: Whoops!

Error objects are thrown when runtime errors occur.
The Error object can also be used as a base object for user-defined exceptions.
See below for standard built-in error types.

发生运行时错误时会抛出错误对象。
Error 对象也可以用作用户定义异常的基础对象。
请参阅下面的标准内置错误类型。

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors

cause

function doWork() {
  try {
    doFailSomeWay();
  } catch (err) {
    throw new Error("Failed in some way", { cause: err });
  }
  try {
    doFailAnotherWay();
  } catch (err) {
    throw new Error("Failed in another way", { cause: err });
  }
}

try {
  doWork();
} catch (err) {
  switch (err.message) {
    case "Failed in some way":
      handleFailSomeWay(err.cause);
      break;
    case "Failed in another way":
      handleFailAnotherWay(err.cause);
      break;
  }
}

The cause data property of an Error instance indicates the specific original cause of the error.
It is used when catching and re-throwing an error with a more-specific or useful error message in order to still have access to the original error.

Error 实例的 cause 数据属性指示错误的具体原始原因。
当使用更具体或有用的错误消息捕获并重新抛出错误时使用它，以便仍然可以访问原始错误。

try {
  connectToDatabase();
} catch (err) {
  console.log(`err =`, err);
  throw new Error('Connecting to database failed.', { cause: err });
}

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/cause

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refs

https://attachments.convertkitcdnn2.com/343082/c6984633-5363-4ca2-b662-47b36693c5b2/understanding-javascript-promises.pdf

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