promise介绍

var promise = new Promise((resolve, reject) => {
    console.log('立即执行');
    var result = 2+1;
    if(result>2){
        resolve(result);                
    }else{
        reject(result);
    }
});

// 写法一
promise.then(value=>{
    console.log('resolve:'+value);
}, e=>{
    console.log('reject:'+e);
});
// 写法二
promise.then(value=>{
    console.log('resolve:'+value);
}).catch(e=>{
    console.log('reject:'+e)
});

promise.then(value=>{
    throw new Error('div 0');
    console.log('resolve:'+value);
}, e=>{ // 捕获不到resovle中的异常
    console.log('reject:'+e);
});
promise.then(value=>{
    throw new Error('div 0');
    console.log('resolve:'+value);
}).catch(e=>{// 可以捕获到resolve中的异常
    console.log('reject:'+e)
});

const makeCancelable = (promise) => {
  let hasCanceled = false;
  const wrappedPromise = new Promise((resolve, reject) => {
    promise.then((val) => {
      const x = hasCanceled ? reject({ isCanceled: true }) : resolve(val);
      return x;
    }).catch((error) => {
      const x = hasCanceled ? reject({ isCanceled: true }) : reject(error);
      return x;
    });
  });
  return {
    promise: wrappedPromise,
    cancel() {
      hasCanceled = true;
    },
  };
};