try...catch

The try...catch statement marks a block of statements to try, and specifies a response, should an exception be thrown.

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Syntax

try {
   try_statements
}
[catch (exception_var_1 if condition_1) { // non-standard
   catch_statements_1
}]
...
[catch (exception_var_2) {
   catch_statements_2
}]
[finally {
   finally_statements
}]

try_statements

The statements to be executed.

catch_statements_1, catch_statements_2

Statements that are executed if an exception is thrown in the try block.

exception_var_1, exception_var_2

An identifier to hold an exception object for the associated catch clause.

condition_1

A conditional expression.

finally_statements

Statements that are executed after the try statement completes. These statements execute regardless of whether an exception was thrown or caught.

Description

The try statement consists of a try block, which contains one or more statements. {} must always be used, also for single statements. At least one catch clause, or a finally clause, must be present. This gives us three forms for the try statement:

1. try...catch
2. try...finally
3. try...catch...finally

A catch clause contains statements that specify what to do if an exception is thrown in the try block. That is, you want the try block to succeed, and if it does not succeed, you want to pass control to the catch block. If any statement within the try block (or in a function called from within the try block) throws an exception, control is immediately shifted to the catch clause. If no exception is thrown in the try block, the catch clause is skipped.

The finally clause executes after the try block and catch clause(s) execute but before the statements following the try statement. It always executes, regardless of whether an exception was thrown or caught.

You can nest one or more try statements. If an inner try statement does not have a catch clause, the enclosing try statement's catch clause is entered.

You can also use the try statement to handle JavaScript exceptions. See the JavaScript Guide for more information on JavaScript exceptions.

Unconditional catch clause

When a single, unconditional catch clause is used, the catch block is entered when any exception is thrown. For example, when the exception occurs in the following code, control transfers to the catch clause.

try {
   throw 'myException'; // generates an exception
}
catch (e) {
   // statements to handle any exceptions
   logMyErrors(e); // pass exception object to error handler
}

The catch block specifies an identifier (e in the example above) that holds the value specified by the throw statement. The catch block is unique in that JavaScript creates this identifier when the catch block is entered and it adds it to the current scope; the identifier lasts only for the duration of the catch block; after the catch block finishes executing, the identifier is no longer available.

Conditional catch clauses

You can also use one or more conditional catch clauses to handle specific exceptions. In this case, the appropriate catch clause is entered when the specified exception is thrown. In the following example, code in the try block can potentially throw three exceptions: TypeError, RangeError, and EvalError. When an exception occurs, control transfers to the appropriate catch clause. If the exception is not one of the specified exceptions and an unconditional catch clause is found, control transfers to that catch clause.

If you use an unconditional catch clause with one or more conditional catch clauses, the unconditional catch clause must be specified last. Otherwise, the unconditional catch clause will intercept all types of exception before they can reach the conditional ones.

Reminder: this functionality is not part of the ECMAScript specification and has been removed in Firefox 59. It's not supported in any current browser anymore.

try {
    myroutine(); // may throw three types of exceptions
} catch (e if e instanceof TypeError) {
    // statements to handle TypeError exceptions
} catch (e if e instanceof RangeError) {
    // statements to handle RangeError exceptions
} catch (e if e instanceof EvalError) {
    // statements to handle EvalError exceptions
} catch (e) {
    // statements to handle any unspecified exceptions
    logMyErrors(e); // pass exception object to error handler
}

Here is the same "Conditional catch clauses" using code that conforms to ECMAScript specification (obviously it's verbose, but works everywhere):

try {
    myroutine(); // may throw three types of exceptions
} catch (e) {
    if (e instanceof TypeError) {
        // statements to handle TypeError exceptions
    } else if (e instanceof RangeError) {
        // statements to handle RangeError exceptions
    } else if (e instanceof EvalError) {
        // statements to handle EvalError exceptions
    } else {
       // statements to handle any unspecified exceptions
       logMyErrors(e); // pass exception object to error handler
    }
}

The exception identifier

When an exception is thrown in the try block, exception_var (e.g. the e in catch (e)) holds the value specified by the throw statement. You can use this identifier to get information about the exception that was thrown. This identifier is local to the catch clause. That is, it is created when the catch clause is entered, and after the catch clause finishes executing, the identifier is no longer available.

function isValidJSON(text) {
    try {
        JSON.parse(text);
        return true;
    } catch {
        return false;
    }
}

The finally clause

The finally clause contains statements to execute after the try block and catch clause(s) execute, but before the statements following the try..catch..finally block. Note that the finally clause executes regardless of whether an exception is thrown. Also, if an exception is thrown, the statements in the finally clause execute even if no catch clause handles the exception.  

You can use the finally clause to make your script fail gracefully when an exception occurs; for example, to do general cleanup, you may need to release a resource that your script has tied up.

It may seem strange to have a special exception-related clause that executes regardless of whether there is an exception, but this construct actually does serve a purpose.  The important point is not that the finally-clause always executes, but rather that ordinary code following a try..catch does not. 

For instance, if another exception occurs inside a try's catch-block, any remaining code in the same outer try-block enclosing that try..catch (or in the main flow, if not in an outer try-block) , will not get executed, since control is immediately transferred to the outer try's catch-block (or the internal error-generator, if not in a try-block).  

Thus, any routine cleanup code done in that enclosed (or the main) section before it exits, will be skipped.  However, If the try-block has a finally-block, then that finally-block code will be executed first to permit any such cleanup, and THEN the other try's catch-block (or the error-generator) will get control to handle the second exception.  

Now, if that routine cleanup must be done whether or not the try..catch code succeeds, then if the finally-block only executed after an exception, the same cleanup code would have to be duplicated both inside and outside the finally-block, and therefore there is no reason not to have just the finally-block alone, and let it execute regardless of exceptions or not.

The following example opens a file and then executes statements that use the file (server-side JavaScript allows you to access files). If an exception is thrown while the file is open, the finally clause closes the file before the script fails. The code in finally also executes upon explicitly returning from try or catch block.

openMyFile();
try {
   // tie up a resource
   writeMyFile(theData);
}
finally {
   closeMyFile(); // always close the resource
}

Examples

Nested try-blocks

First, let's see what happens with this:

try {
  try {
    throw new Error('oops');
  }
  finally {
    console.log('finally');
  }
}
catch (ex) {
  console.error('outer', ex.message);
}

// Output:
// "finally"
// "outer" "oops"

Now, if we already caught the exception in the inner try-block by adding a catch block

try {
  try {
    throw new Error('oops');
  }
  catch (ex) {
    console.error('inner', ex.message);
  }
  finally {
    console.log('finally');
  }
}
catch (ex) {
  console.error('outer', ex.message);
}

// Output:
// "inner" "oops"
// "finally"

And now, let's re-throw the error.

try {
  try {
    throw new Error('oops');
  }
  catch (ex) {
    console.error('inner', ex.message);
    throw ex;
  }
  finally {
    console.log('finally');
  }
}
catch (ex) {
  console.error('outer', ex.message);
}

// Output:
// "inner" "oops"
// "finally"
// "outer" "oops"

Any given exception will be caught only once by the nearest enclosing catch-block unless it is re-thrown. Of course, any new exceptions raised in the "inner" block (because the code in catch-block may do something that throws), will be caught by the "outer" block.

Returning from a finally block

If the finally block returns a value, this value becomes the return value of the entire try-catch-finally production, regardless of any return statements in the try and catch blocks. This includes exceptions thrown inside of the catch block:

(function() {
  try {
    try {
      throw new Error('oops');
    }
    catch (ex) {
      console.error('inner', ex.message);
      throw ex;
    }
    finally {
      console.log('finally');
      return;
    }
  }
  catch (ex) {
    console.error('outer', ex.message);
  }
})();

// Output:
// "inner" "oops"
// "finally"

The outer "oops" is not thrown because of the return in the finally block. The same would apply to any value returned from the catch block.

Specifications

Browser compatibility

See also

• Error
• throw