How to detect errors from contract calls

Question

for (var i = 0 ; i <= 4 ; i++) {
  myContract.methods.contractmanager(i).call().then((res) => {
    console.log(res[0]);
  }):
}

There are 2 data stored in my contract. I want to print it but it is not always 2, sometimes there will be more. It is dynamic and I wont know how many there will be.

After executing this code I have 2 output and 2 error. How do I make the loop stop at 2 so it wont show the errors? How do I detect the error and break the loop? How do I find out if the output is empty or not?

Answer 1

You can use async/await.

(async function () {
    for await (let i of Array.from(Array(5).keys())) {
        let result= await  myContract.methods.contractmanager(i).call();
        console.log(result);
    }      
})();

Answer 2

If you want to break the loop when you detect an error then you should use async and await. For example:

const main = async () => {
  // ... the rest of your code before this

  for (let i = 0; i <= 4; i++) {
    try {
      const result = await myContract.methods.contractmanager(i).call()
      // handle the result
    } catch (err) {
      break // or handle the error however you'd like
    }
  }
}

main()

Answer 3

You can check the output type using typeof method. Let's say the datatype of the two output is string:

(async () => {
     for(var i = 0 ; i <= 4 ; i++){
       const res = await myContract.methods.contractmanager(i).call();
         if(typeof res=="object" && typeof res[0]=="string" && && typeof res[1]=="string") {
            console.log(res[0], res[1]);
         } else break;
    }
})()