Promise.js

function Promise(exec) {
  const self = this;
  self.status = "pending";
  self.value = undefined;
  self.onResolvedCallbacks = [];
  self.onRejectedCallbacks = [];

  function resolve(value) {
    if (self.status === "pending") {
      setTimeout(() => {
        self.status = "resolved";
        self.value = value;
        while ((cb = self.onResolvedCallbacks.shift())) {
          cb(value);
        }
      });
    }
  }
  function reject(reason) {
    if (self.status === "pending") {
      setTimeout(() => {
        self.status = "rejected";
        self.value = reason;
        while ((cb = self.onRejectedCallbacks.shift())) {
          cb(reason);
        }
      });
    }
  }
  try {
    exec(resolve, reject);
  } catch (e) {
    reject(e);
  }
}
function empty(v) {
  return v
}
Promise.prototype.then = function(onResolve = empty, onReject = empty) {
  const self = this;
  const { status, value } = self;
  if (status === "resolved") {
    return new Promise(function(resolve, reject) {
      try {
        const res = onResolve(value);
        if (res instanceof Promise) {
          res.then(resolve, reject);
        } else {
          resolve(res);
        }
      } catch (e) {
        reject(e);
      }
    });
  } else if (status === "rejected") {
    return new Promise(function(resolve, reject) {
      try {
        const res = onReject(value);
        if (res instanceof Promise) {
          res.then(resolve, reject);
        } else {
          reject(res);
        }
      } catch (e) {
        reject(e);
      }
    });
  } else {
    return new Promise(function(resolve, reject) {
      self.onResolvedCallbacks.push(function(v) {
        try {
          const res = onResolve(v);
          if (res instanceof Promise) {
            res.then(resolve, reject);
          } else {
            resolve(res);
          }
        } catch (e) {
          reject(e);
        }
      });

      self.onRejectedCallbacks.push(function(reason) {
        try {
          const res = onReject(reason);
          if (res instanceof Promise) {
            x.then(resolve, reject);
          } else {
            reject(res);
          }
        } catch (e) {
          reject(e);
        }
      });
    });
  }
};
Promise.prototype.catch = function (onReject) {
  return this.then(undefined,onReject)
}

var getData100 = function() {
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      resolve("100ms");
    }, 1000);
  });
};

var getData200 = function() {
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      resolve("200ms");
    }, 2000);
  });
};
var getData300 = function() {
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      reject("reject");
    }, 3000);
  });
};

getData100()
  .then(function(data) {
    console.log(data); // 100ms
    return getData200();
  })
  .then(function(data) {
    console.log(data); // 200ms
    return getData300();
  })
  .then(
    function(data) {
      console.log(data); // 100ms
    },
    function(data) {
      console.log(data);
      // throw new Error('boom!')
    }
  ).catch(e => {
    console.log(e)
  })
  ;
// var m = new Promise((resolve, reject) => {
//   resolve(1);
// });
// m.then(() => {
//   console.log(1);
// });
// m.then(() => {
//   console.log(2);
// });
// console.log("sync");