关于try..catch..finally..throw

try
每个try块必须与至少一个catch或finally块，否则会抛出SyntaxError错误。
单独使用try块进行验证：

try {
  throw new Error('Error while executing the code');
}
1
2
3
ⓧ Uncaught SyntaxError: Missing catch or finally after try
1
try…catch
将try与catch块一起使用，可以处理try块抛出的错误。

try {
  throw new Error('Error while executing the code');
} catch (err) {
  console.error(err.message);
}

ⓧ Error while executing the code
try…catch 与 无效代码
try…catch 无法捕获无效的 JS 代码，例如try块中的以下代码在语法上是错误的，但它不会被catch块捕获。

try {
  ~!$%^&*
} catch(err) {
  console.log("这里不会被执行");
}
 ⓧ Uncaught SyntaxError: Invalid or unexpected token
1
try…catch 与 异步代码
try…catch无法捕获在异步代码中引发的异常，例如setTimeout：

try {
  setTimeout(function() {
    noSuchVariable;   // undefined variable
  }, 1000);
} catch (err) {
  console.log("这里不会被执行");
}

未捕获的ReferenceError将在1秒后引发:

ⓧ Uncaught ReferenceError: noSuchVariable is not defined
应该在异步代码内部使用 try…catch 来处理错误：

setTimeout(function() {
  try {
    noSuchVariable;
  } catch(err) {
    console.log("error is caught here!");
  }
}, 1000);
嵌套 try…catch
使用嵌套的try和catch块向上抛出错误

try {
  try {
    throw new Error('Error while executing the inner code');
  } catch (err) {
    throw err;
  }
} catch (err) {
  console.log("Error caught by outer block:");
  console.error(err.message);
}
Error caught by outer block:
 ⓧ Error while executing the code
1
2
try…finally
仅使用 try…finally 而没有 catch 块

try {
  throw new Error('Error while executing the code');
} finally {
  console.log('finally');
}
finally
ⓧ Uncaught Error: Error while executing the code
注意

即使从try块抛出错误后，也会执行finally块
如果没有catch块，错误将不能被优雅地处理，从而导致未捕获的错误
try…catch…finally
使用try…catch块和可选的finally块

try {
  console.log("Start of try block");
  throw new Error('Error while executing the code');
  console.log("End of try block -- never reached");
} catch (err) {
  console.error(err.message);
} finally {
  console.log('Finally block always run');
}
console.log("Code execution outside try-catch-finally block continue..");
Start of try block
ⓧ Error while executing the code
Finally block always run
Code execution outside try-catch-finally block continue..
注意

在try块中抛出错误后往后的代码不会被执行了
即使在try块抛出错误之后，finally块仍然执行
finally块通常用于清理资源或关闭流

try {
  openFile(file);
  readFile(file);
} catch (err) {
  console.error(err.message);
} finally {
  closeFile(file);
}
throw
throw语句用于引发异常

// throw primitives and functions
throw "Error404";
throw 42;
throw true;
throw {toString: function() { return "I'm an object!"; } };

// throw error object
throw new Error('Error while executing the code');
throw new SyntaxError('Something is wrong with the syntax');
throw new ReferenceError('Oops..Wrong reference');

// throw custom error object
function ValidationError(message) {
  this.message = message;
  this.name = 'ValidationError';
}
throw new ValidationError('Value too high');