Nodejs Promise() error swallowing

Question

I am struggling with promises

having the following snippet:

Promise.resolve()
    .then(a("OK1"))
    .then(a("OK2"))
    .then(a("OK3"))
    .then(a("OK4"))
    .then(a("OK5"))
    .then(function() {
        console.log("FINISH");
    })
    .catch(function(e) {
      
        console.log("ERROR: " + e);
    });


function a(i) {
    return new Promise(function(resolve, reject) {
        
        if(i == "OK4") {
           console.log("THROW");
           throw('Error'); //This does not happen
           reject();
           return;
        }
        if(i == "OK2") {
          reject()
          return;
        }
        console.log(i);
        resolve();
    });
}

i want to be able to reject a promise - then the next .then is executed ("OK3") but i am unable to throw exceptions - i want to trigger the .catch on "OK4"

OK4 - is rejected and not outputted, but OK5 still gets executed - is there a way arround it?

this would be my expected output:

OK1
OK3
ERROR: ....

Answer 1

正如Bergi所提到的，您在每个.then上传递的参数会立即运行该函数并返回一个promise。 promise链需要引用它可以在以后运行的函数。

如果要使用参数运行函数，可以将其包装在匿名函数中，以便promise链可以在以后运行匿名函数。

a("OK1")
  .then((res)=>{
    return a("OK2")
  })
  .then((res)=>{
    return a("OK3")
  })
  .then((res)=>{
    return a("OK4")
  })
  .then((res)=>{
    return a("OK5")
  })
  .then(()=>{
    console.log("FINISH");
  })
  .catch(function(e) {
    console.log("ERROR: " + e);
  });


function a (i) {
    return new Promise(function(resolve, reject) {

        if(i == "OK4") {
           console.log("THROW");
           throw new Error(i); //This does not happen
        }
        if(i == "OK2") {
          return reject(new Error(i))
        }
        setTimeout(function () {
          console.log(i);
          resolve(i); 
        }, 100)
    });
}

传递一个最终会返回一个promise的函数是可以的。该函数将传递前一个promise解析的值。

b(1)
  .then(b)
  .then(b)
  .then(b)
  .then((res) => {
    console.log("FINISH",res);
  })
  .catch(function (e) {
    console.log("ERROR: " + e);
  });

function b (i) {
  return new Promise(function (resolve, reject) {
    console.log(i)
    resolve(i+1)
  })
}