CODEWITHSUNDEEP
javascriptasynchronouspromise
Gayathri
Gayathri

Reputation: 1906

Throw error based on result of promise

The below code doesn't handle error though condition is satisfied. Please help understand why

Edit:the below code now terminates code flow in case of satisfying condition but throwing a error result in unhandled rejection.

utils.js

const isMaximum = Id => {
 return db.Entries
   .findAndCountAll()
   .then(counts => {
     let numberOfEntries = 2;
     let maxEntries = 2;
       if (numberOfEntries == maxEntries) {
         return true;
       }
       else{
         return false;
       }
     });
};

xyz.js

const doSomething=(req,res,next)=>{
    Utils.isMaximum(id).then(isLimit=> {

           if(isLimit) throw new Error("not allowed"); //unhandled rejection
         //rest of code gets terminated as expected
 });
}

Related questions Why can I not throw inside a Promise.catch handler? couldn't find me a solution. So please make it clear.

Upvotes: 0

Views: 59

Answers (2)

jfriend00
jfriend00

Reputation: 708206

Throwing inside a .catch() handler makes the promise chain a rejected promise. If you don't have a subsequent .catch() handler, then it will get reported as an unhandled promise rejction because you have no .catch() afterwards to catch the rejected promise. This is nearly always a programming error which is why you get the warning.

Throwing inside a .catch() does not terminate your program. That exception is caught by the promise infrastructure (and turned into a rejected promise). That's how promises work (by specification). If you want your program to terminate at that point, then you can call process.exit() rather than throwing.

Now you've edited your question and now you're trying to throw inside a .then() handler. OK. I'll address what you've added. Next time please start with the whole code context so we can more quickly understand the situation.

Utils.isMaximum() is asynchronous. It returns a promise. You need to program with using asynchronous techniques. You can't think like synchronous sequential code. Any code that comes after your call to Utils.isMaximum() will execute BEFORE the .then() handler is called.

Here's a simple example:

Utils.isMaximum(id).then(isLimit => {
   console.log("A");
});
console.log("B");

That will output:

B
A

So, for starters, you can't prevent the code that comes after the .then() block from running by anything you do inside the .then() block because the rest of the function executes BEFORE the .then() handler does.

So, the usual way you deal with that is you have to put the rest of the code INSIDE the .then() handler or inside a function you call from there:

const doSomething=(req,res,next)=>{
    Utils.isMaximum(id).then(isLimit=> {
         if(!isLimit){
               // rest of code here that sends regular response
         } else {
               // handle error here - send error response
               // or OK to throw here, throw will be caught by next .catch()
         }
    }).catch(err => {
          // send error response here
    });
}

Upvotes: 2

Gayathri
Gayathri

Reputation: 1906

I didn't catch the error I throw and that is the only reason why. Below resolved.

const doSomething=(req,res,next)=>{
    Utils.isMaximum(id).then(isLimit=> {

           if(isLimit) throw new Error("not allowed"); 
 }).catch(next);
   //rest of code gets terminated as expected

}

Upvotes: 0

Related Questions