freeCodeCamp/guide/english/javascript/error-handling-and-try-catch-throw/index.md

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Error Handling and Try Catch Throw

Error Handling and Try Catch Throw

The try...catch..finally statement marks a block of statements to try, and specifies a response, should an exception be thrown. The try statement contains one or more statements, and at least one catch clause or a finally clause, or both.

try...catch:

try {
   throw new Error('my error');
}
catch (e) {
  console.error(e.message);
}

// Output:
// my error

try...finally:

try {
   throw new Error('my error');
}
finally {
  console.error('finally');
}

// Output:
// finally

Note: when you don't catch the error, it is in fact not 'catched', even though the finally block is executed. That means that the error will continue to the upper try block (or main block).

try...catch...finally:

try {
   throw new Error('my error');
}
catch (e) {
  console.error(e.message);
}
finally {
  console.error('finally');
}

// Output:
// my error
// finally

Typical usage:

try {
   openFile(file);
   readFile(file)
}
catch (e) {
  console.error(e.message);
}
finally {
  closeFile(file);
}

Nested try...catch:

You can also:

• Nest a try-catch statement inside a try block.
• Throw the error upwards.

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

// Output:
// inner my error
// finally
// outer my error

Types of Error

Reference Error

var x;
try {
  x = y + 1;   // y cannot be referenced (used)
}
catch(err) {
  console.log(err.name, err.message);
}
// ReferenceError y is not defined

Syntax Error

try {
    eval("alert('Hello)");   // Missing ' will produce an error
}
catch(err) {
    console.log(err.name,err.message);
}
// SyntaxError Invalid or unexpected token

Type Error

var num = 1;
try {
    num.toUpperCase();   // You cannot convert a number to upper case
}
catch(err) {
    console.log(err.name, err.message);
}
// TypeError num.toUpperCase is not a function

URI Error

try {
    decodeURI("%%%");   // You cannot URI decode these percent signs
}
catch(err) {
    console.log(err.name, err.message);
}
// URIError URI malformed

More Information:

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