Javascript try catch

Learn how to handle error in javascript with try, catch and finally.

No matter how good you have programmed your application there is always a chance that something may result in error which can break your application. You can minimize this with use of try & catch in javascript.

Syntax

try{
  //code to be executed which may result in error
}catch(error){
  //code to handle error
}finally{
  //code to be executed regardless of try/catch result
}

try

In try statement you can execute block of codes that can result in error.

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catch(err)

In catch statement you can handle the error that is thrown while executing the code inside try. It is not necessary to declare catch block but it is a good practice to do so.

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err

Required if catch is used to know exactly what error is thrown.

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finally

To run block of code irrespective of what happens in try and catch. It is not necessary to declare this block you can use it as per need.

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Example

 try{
   if((3 + 2) !== 4){
      throw 'Not equal to 4';
   }else{
      console.log('2 + 2 is 4');
   }
}catch(err){
  console.log('There is new error ' + err);
}

//"There is new error Not equal to 4"

Using finally

try{
   if((2 + 2) !== 4){
     throw 'Not equal to 4';
   }else{
     console.log('2 + 2 is 4');
   }
}catch(err){
   console.log('There is new error ' + err);
}finally{
   console.log('I don't really care whether 2 + 2 is 4 or not');
}

//"2 + 2 is 4"
//"I dont really care whether 2 + 2 is 4 or not"

catch & finally both are optional and can be avoided.

try with finally

try{
  if((2 + 2) === 4){
    console.log('I was right');
  }
}finally{
   console.log('I am finally free');
}

//"I was right"
//"I am finally free"

Javascript does not allow multiple catch statements, however you can use logical conditioning like if else or switch case to handle this type of situations.

try{
   if((2 + 2) !== 4){
      throw new Error('It is not equal to 4');
   }else{
      return true;
   }
}catch(err){
  if(err instanceOf TypeError){
    console.log(`There is new TypeError ${err}`);
  }else if(err instanceOf RangeError){
    console.log(`There is new RangeError ${err}`);
  }else{
    console.log(`There is new error ${err}`);
  }
}

You can also use nested try & catch if you want.

Handling error with single catch.

try{
  try{
    throw 'I was inside the nested try at level 2';
  }finally{
    console.log('I will execute no matter what');
  }
}catch(err){
  console.log(`There is a error caught at level 1:-  ${err}`);
}

//"I will execute no matter what"
//"There is a error caught at level 1:-  I was inside the nested try at level 2"

Parent catch will handle all the error which are thrown by it’s children try and are not handled at their level.

Handling error with nested catch.

try{
  try{
    throw 'I was inside the nested try at level 2';
  }catch(err){
    console.log(`There is a error caught at level 2:-  ${err}`);  
  }finally{
    console.log('I will execute no matter what');
  }
}catch(err){
  console.log(`There is a error caught at level 1:-  ${err}`);
}

//"There is a error caught at level 2:-  I was inside the nested try at level 2"
//"I will execute no matter what"

Error was handled by the nested catch only.

Re throwing the error with nested catch.

try{
  try{
    throw 'I was inside the nested try at level 2';
  }catch(err){
    console.log(`There is a error caught at level 2:-  ${err}`);
    throw 'I was inside the nested catch at level 2';  
  }finally{
    console.log('I will execute no matter what');
  }
}catch(err){
  console.log(`There is a error caught at level 1:-  ${err}`);
}

//"There is a error caught at level 2:-  I was inside the nested try at level 2"
//"I will execute no matter what"
//"There is a error caught at level 1:-  I was inside the nested catch at level 2"

Checkout the sequence of execution here.

1. First the code inside the nested try at level 2 is executed.
2. Then error is handled by the level 2 catch and it re – throws the error.
3. Now the code inside the Finally is executed.
4. At last the parent catch statement handles the error thrown inside the level 2 catch.