Cliffy

Error and exit handling

Cliffy throws an ValidationError for invalid options, arguments and environment variables. ValidationError’s can be also throw manually. By default, when a ValidationError is thrown, cliffy prints the auto generated help and the error message and calls Deno.exit(validationError.exitCode ?? 1) to exit the program. This behaviour can be changed by calling .throwErrors() or .noExit() or by adding an error handler.

Throw errors

By default, cliffy prints the help text of the failed command together with the error message and calls Deno.exit() when a ValidationError is thrown. All other errors will be thrown by default. You can override this behaviour with the .throwErrors() method to always throw errors.

No exit

The .noExit() method does the same as .throwErrors() but also prevents the command from calling Deno.exit() for example when the help or version option is called.

Error handler

Errors can be caught by simply wrapping the .parse() method into a try catch block. But you can also register an error handler with the .error() method.

Child commands can override error handlers from parent commands. If the error handler doesn’t throw or call Deno.exit, the default error handler is executed.

The first argument of the error handler is the error and the second argument the instance of the failed command. You can use this instance to print the help text from this command.

import {
  Command,
  ValidationError,
} from "https://deno.land/x/cliffy@v0.25.7/command/mod.ts";

await new Command()
  .error((error, cmd) => {
    if (error instanceof ValidationError) {
      cmd.showHelp();
    }
    console.error(error);
    Deno.exit(error instanceof ValidationError ? error.exitCode : 1);
  })
  .action(() => {
    throw new ValidationError("validation error message.");
  })
  .parse();Copy

Runtime errors

This example will catch only runtime errors.

import { Command } from "https://deno.land/x/cliffy@v0.25.7/command/mod.ts";

const cmd = new Command()
  .option("-p, --pizza-type <type>", "Flavour of pizza.")
  .action(() => {
    throw new Error("Some error happened.");
  });

try {
  await cmd.parse();
} catch (error) {
  console.error("[CUSTOM_ERROR]", error);
  Deno.exit(1);
}Copy
$ deno run https://deno.land/x/cliffy@v0.25.7/examples/command/general_error_handling.ts -t
Unknown option "-t". Did you mean option "-h"?
$ deno run https://deno.land/x/cliffy@v0.25.7/examples/command/general_error_handling.ts
[CUSTOM_ERROR] Some error happened.Copy

Validation errors

This example will catch all errors. You can differentiate between runtime and validation errors by checking if the error is an instance of ValidationError. The validation error has a exitCode property that should be used to exit the program. It also provides a cmd property which references the failed command.

import {
  Command,
  ValidationError,
} from "https://deno.land/x/cliffy@v0.25.7/command/mod.ts";

const cmd = new Command()
  .throwErrors() // <-- throw also validation errors.
  .option("-p, --pizza-type <type>", "Flavour of pizza.")
  .action(() => {
    throw new Error("Some error happened.");
  });

try {
  await cmd.parse();
} catch (error) {
  if (error instanceof ValidationError) {
    error.cmd?.showHelp();
    console.error("Usage error: %s", error.message);
    Deno.exit(error.exitCode);
  } else {
    console.error("Runtime error: %s", error);
    Deno.exit(1);
  }
}Copy
$ deno run https://deno.land/x/cliffy@v0.25.7/examples/command/validation_error_handling.ts -t
Usage error: Unknown option "-t". Did you mean option "-h"?Copy

Custom validation errors

You can throw custom validation errors by throwing an instance of ValidationError. Optional you can define the exit code that should be used when Deno.exit() is called after displaying the auto generated help and the error message.

import {
  Command,
  ValidationError,
} from "https://deno.land/x/cliffy@v0.25.7/command/mod.ts";

await new Command()
  .option("-c, --color <name:string>", "Choose a color.")
  .action(({ color }) => {
    if (color === "black") {
      throw new ValidationError("Black is not supported.", { exitCode: 1 });
    }
  })
  .parse();Copy

Throw errors outside the command context

The .throw() method can be used to throw errors outside the command context so you have the same behaviour as when you throw an error for example within an action handler or a type.

import {
  Command,
  ValidationError,
} from "https://deno.land/x/cliffy@v0.25.7/command/mod.ts";

const { options, cmd } = await new Command()
  .error((_error, _cmd) => {
    console.error("error handler...");
    // Throw or call Deno.exit() to disable the default error handler.
    // throw _error;
    // Deno.exit(_error instanceof ValidationError ? _error.exitCode : 1);
  })
  .option("-r, --runtime-error", "Triggers a runtime error.")
  .option("-v, --validation-error", "Triggers a validation error.")
  .parse();

if (options.validationError) {
  cmd.throw(new ValidationError("validation error message."));
}

if (options.runtimeError) {
  cmd.throw(new Error("runtime error message."));
}Copy
$ deno run https://deno.land/x/cliffy@v0.25.7/examples/command/throw.ts --runtime-error
error handler...
error: Uncaught Error: runtime error message.
  cmd.throw(new Error("runtime error message."));
            ^
    at https://deno.land/x/cliffy@v0.25.7/examples/command/throw.ts:21:13Copy
$ deno run https://deno.land/x/cliffy@v0.25.7/examples/command/throw.ts --validation-error
error handler...

  Usage: COMMAND

  Options:

    -h, --help              - Show this help.
    -r, --runtime-error     - Triggers a runtime error.
    -v, --validation-error  - Triggers a validation error.

  error: validation error message.Copy
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