Promises — الحالات والسلاسل ومعالجة الأخطاء

مقدمة إلى Promise

حل Promise

// Create a Promise and resolve it
function waitMs(ms) {
  return new Promise(function (resolve) {
    setTimeout(function () {
      resolve("done");
    }, ms);
  });
}

console.log("Start");
waitMs(10).then(function (msg) {
  console.log("Resolved:", msg);
});

الرفض والتقاط الخطأ

// Reject to signal an error; handle with catch
function maybeFail(ok) {
  return new Promise(function (resolve, reject) {
    if (ok) {
      resolve("ok");
    } else {
      reject(new Error("bad"));
    }
  });
}

maybeFail(true)
  .then(function (v) {
    console.log("A:", v);
    return maybeFail(false);
  })
  .catch(function (e) {
    console.log("Caught:", e.message);
  });

أساسيات تسلسل then

// Chain then steps; return values flow to the next then
Promise.resolve(2)
  .then(function (n) {
    return n * 3;
  })
  .then(function (n) {
    console.log("Result:", n);
  });

التنظيف باستخدام finally

// finally runs whether fulfilled or rejected
function task(ok) {
  return new Promise(function (resolve, reject) {
    ok ? resolve("yay") : reject(new Error("oops"));
  });
}

task(false)
  .catch(function (e) {
    console.log("Handled:", e.message);
  })
  .finally(function () {
    console.log("Cleanup done");
  });

لمحة عن Promise.all

// Promise.all waits for all; rejects fast if any fail
const a = waitMs(5);
const b = Promise.resolve("B");

Promise.all([a, b])
  .then(function (values) {
    console.log("All:", values);
  })
  .catch(function (e) {
    console.log("All failed:", e.message);
  });

اختبار معالجة أخطاء Promise

مراجعة