try...catch

try {
  tryStatements
} catch (exceptionVar) {
  catchStatements
} finally {
  finallyStatements
}

tryStatements

The statements to be executed.

catchStatements

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

exceptionVar Optional

An optional identifier or pattern to hold the caught exception for the associated catch block. If the catch block does not use the exception's value, you can omit the exceptionVar and its surrounding parentheses.

finallyStatements

Statements that are executed before control flow exits the try...catch...finally construct. These statements execute regardless of whether an exception was thrown or caught.

The try statement always starts with a try block. Then, a catch block or a finally block must be present. It's also possible to have both catch and finally blocks. This gives us three forms for the try statement:

• try...catch
• try...finally
• try...catch...finally

Unlike other constructs such as if or for, the try, catch, and finally blocks must be blocks, instead of single statements.

try doSomething(); // SyntaxError
catch (e) console.log(e);

A catch block contains statements that specify what to do if an exception is thrown in the try 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 block. If no exception is thrown in the try block, the catch block is skipped.

The finally block will always execute before control flow exits the try...catch...finally construct. 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 block, the enclosing try statement's catch block is used instead.

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

When an exception is thrown in the try block, exceptionVar (i.e., the e in catch (e)) holds the exception value. You can use this binding to get information about the exception that was thrown. This binding is only available in the catch block's scope.

It doesn't need to be a single identifier. You can use a destructuring pattern to assign multiple identifiers at once.

try {
  throw new TypeError("oops");
} catch ({ name, message }) {
  console.log(name); // "TypeError"
  console.log(message); // "oops"
}

The bindings created by the catch clause live in the same scope as the catch block, so any variables declared in the catch block cannot have the same name as the bindings created by the catch clause. (There's one exception to this rule, but it's a deprecated syntax.)

try {
  throw new TypeError("oops");
} catch ({ name, message }) {
  var name; // SyntaxError: Identifier 'name' has already been declared
  let message; // SyntaxError: Identifier 'message' has already been declared
}

The exception binding is writable. For example, you may want to normalize the exception value to make sure it's an Error object.

try {
  throw "Oops; this is not an Error object";
} catch (e) {
  if (!(e instanceof Error)) {
    e = new Error(e);
  }
  console.error(e.message);
}

If you don't need the exception value, you can omit it along with the enclosing parentheses.

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

The finally block contains statements to execute after the try block and catch block(s) execute, but before the statements following the try...catch...finally block. Control flow will always enter the finally block, which can proceed in one of the following ways:

• Immediately after the try block finishes execution normally (and no exceptions were thrown);
• Immediately after the catch block finishes execution normally;
• Immediately before the execution of a control-flow statement (return, throw, break, continue) in the try block or catch block that would exit the block.

If an exception is thrown from the try block, even when there's no catch block to handle the exception, the finally block still executes, in which case the exception is still thrown immediately after the finally block finishes executing.

The following example shows one use case for the finally block. The code opens a file and then executes statements that use the file; the finally block makes sure the file always closes after it is used even if an exception was thrown.

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

Control flow statements (return, throw, break, continue) in the finally block will "mask" any completion value of the try block or catch block. In this example, the try block tries to return 1, but before returning, the control flow is yielded to the finally block first, so the finally block's return value is returned instead.

function doIt() {
  try {
    return 1;
  } finally {
    return 2;
  }
}

doIt(); // returns 2

It is generally a bad idea to have control flow statements in the finally block. Only use it for cleanup code.

When a catch block is used, the catch block is executed when any exception is thrown from within the try block. For example, when the exception occurs in the following code, control transfers to the catch block.

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 of the exception; this value is only available in the scope of the catch block.

You can create "Conditional catch blocks" by combining try...catch blocks with if...else if...else structures, like this:

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
  }
}

A common use case for this is to only catch (and silence) a small subset of expected errors, and then re-throw the error in other cases:

try {
  myRoutine();
} catch (e) {
  if (e instanceof RangeError) {
    // statements to handle this very common expected error
  } else {
    throw e; // re-throw the error unchanged
  }
}

This may mimic the syntax from other languages, like Java:

try {
  myRoutine();
} catch (RangeError e) {
  // statements to handle this very common expected error
}
// Other errors are implicitly re-thrown

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);
}

// Logs:
// "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);
}

// Logs:
// "inner" "oops"
// "finally"

And now, let's rethrow 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);
}

// Logs:
// "inner" "oops"
// "finally"
// "outer" "oops"

Any given exception will be caught only once by the nearest enclosing catch block unless it is rethrown. 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.

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

(() => {
  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);
  }
})();

// Logs:
// "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.

• Error
• throw