# Error handling

## Function
 error handling is used to actively throw errors in the script and continue execution after being caught in the outer layer. `throw` immediately interrupts the current flow of execution until caught by the nearest `catch`; if there are no `catch`s, the program terminates with an uncaught exception.

## Syntax
```bt
try {
 // Codes that may throw errors
} catch e {
 // e is the error message
}

throw 'fatal'
```

## Parameters
- `try`: Wrap the code block that needs to catch the error.
- `catch e`: Catch errors thrown in `try`, where `e` is the name of the variable that receives the error message.
- `throw value`: Throws the value of any expression. A common usage is to throw a string error message.

## Return Value
`try/catch` can also be used as an expression. When no error is thrown, the value of the last statement of the `try` code block is returned; when an error is caught, the value of the last statement of the `catch` code block is returned.

## Code Examples
```bt
result = try {
    throw 'fatal'
    'ok'
} catch e {
    'caught: ' + e
}

// Output: catch: fatal
print result
```

## Notes
- The sibling code following `throw` will not continue to execute.
 - `throw` can propagate outward through function calls until captured by the outer `catch`.
- `catch` variables are treated as ordinary variable variables; currently BT does not have block-level scope, so do not reuse confusing variable names in the same scope.
 - An uncaught `throw` terminates the program with an uncaught exception message.