Try catch

try...catch

try...catch Statement marks the statement block to try , And specify a response thrown when an exception occurs .

grammar

try {
  //  Statements that need to be executed .
  //  try_statements
}
//  If in try The statement executed when an exception is thrown in the block .
catch (exception) {
  //  catch_statements
}
//  stay try A statement block executed after a statement block . Whether or not there is an exception thrown or caught, these statements will be executed .
finally {
  //  finally_statements
}

describe

try A statement consists of one or more statements try block , And at least one catch A piece or a finally One of the blocks , Or both , Here are three forms of try Statement ：

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

catch Clause contains try The statement to be executed when an exception is thrown in the block . That is to say , You want try The content in the statement succeeded , If it doesn't work , You want to control what happens next , At this time, you can catch Statement . If in try Any statement in the block （ Or from try Block ） Throw an exception , Control immediate steering catch Clause . If in try No exception is thrown in the block , Will skip catch Clause .

finally Clause in try Block and catch Execute after block, but in the next try Execute before declaration . Whether or not an exception is thrown or caught, it always executes .

You can nest one or more try sentence . If inside try The statement did not catch Clause , Then it will enter the package try Of the statement catch Clause .

Conditions catch block

You can also use one or more conditions catch Clause to handle specific exceptions . under these circumstances , When an exception is thrown, the appropriate catch clause . In the following code ,try Block code may throw three exceptions ：TypeError,RangeError and EvalError. When an exception is thrown , The control will enter its corresponding catch sentence . If the exception is not specific , Then control will shift to unconditional catch Clause .

When using an unconditional catch Clause and one or more conditional statements , Unconditional catch Clause must be placed last . Otherwise, all exceptions before reaching the conditional statement will be intercepted by the non conditional statement .

Do not conform to the ECMAscript Examples of specifications

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
}

accord with ECMAscript Examples of specifications

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

nesting try block

Example 1

try {
  try {
    // throw  Statement throws an error   When a mistake happens JavaScript Will stop and throw an error message 
    throw new Error('oops');
  }
  finally {
    console.log('finally');
  }
}
catch (e) {
  console.error('outer', e.message);
}
// Output:
// 'finally'
// 'outer' 'oops'

Example 2

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

//  Any given exception will only be closed by the nearest one  catch  Block capture once . Of course , stay “inner” Any new exceptions thrown by the block  （ because  catch  The code in the block can also throw exceptions ）, Will be “outer” The block captures .