feat: do not print errors returned from yargs (#300)

README.md

@@ -34,6 +34,7 @@ intuitive to work with.
 - [Getting started](#getting-started)
   - [Installation](#installation)
   - [Simple example](#simple-example)
+  - [Error handling](#error-handling)
   - [REPL example](#repl-example)
   - [Prompt](#prompt)
   - [TypeScript](#typescript)
@@ -124,6 +125,58 @@ Options:
 _ℹ You see `bin.js` here instead of `foo.ts` because we're running the program
 with `ts-node`._
 
+### Error handling
+
+We first create a new program called `cat.ts` which is a simple version of the
+`cat` program we all know:
+
+```ts
+import { readFileSync } from 'fs'
+import { program, command } from 'bandersnatch'
+
+const cat = command('cat')
+  .description('Concatenate files')
+  .argument('files', { variadic: true })
+  .action(({ files }) =>
+    console.log(
+      files.reduce((str, file) => str + readFileSync(file, 'utf8'), '')
+    )
+  )
+
+program().default(cat).run()
+```
+
+Now try your program by running it:
+
+```
+$ ts-node cat.ts somefile
+contents of somefile
+```
+
+However, when `somefile` doesn't exist, we get are faced with an ugly unhandled
+promise rejection warning/error (depending on the Node.js version you're using).
+
+Let's fix that:
+
+```diff
+-program().default(cat).run()
++program()
++  .default(cat)
++  .run()
++  .catch((err) => {
++    console.error(`There was a problem running this command:\n${String(err)}`)
++    process.exit(1)
++  })
+```
+
+Which will yield:
+
+```
+$ ts-node cat.ts somefile
+There was a problem running this command:
+Error: ENOENT: no such file or directory, open 'somefile'
+```
+
 ### REPL example
 
 A program can also show an interactive

examples/cat.ts

@@ -0,0 +1,19 @@
+import { readFileSync } from 'fs'
+import { command, program } from '../src'
+
+const cat = command('cat')
+  .description('Concatenate files')
+  .argument('files', { variadic: true })
+  .action(({ files }) =>
+    console.log(
+      files.reduce((str, file) => str + readFileSync(file, 'utf8'), '')
+    )
+  )
+
+program()
+  .default(cat)
+  .run()
+  .catch((err) => {
+    console.error(`There was a problem running this command:\n${String(err)}`)
+    process.exit(1)
+  })

examples/errors.ts

@@ -1,4 +1,4 @@
-import { program, command } from '../src'
+import { command, program } from '../src'
 
 const app = program()
 

src/program.ts

@@ -58,12 +58,12 @@ type ProgramOptions = {
 
   /**
    * Specifies whether to add a default behaviour for an `exit` command.
-   * 
+   *
    * Takes a boolean or a function argument:
    * - `false` installs no handler
    * - `true` will install the default handler
    * - a given function will be installed as the handler
-   * 
+   *
    * Defaults to `() => process.exit()`.
    */
   exit?: boolean | (() => void)
@@ -99,7 +99,10 @@ export class Program extends (EventEmitter as new () => TypedEventEmitter<Events
     }
 
     // Set default exit handler
-    if (this.options.exit === true || typeof this.options.exit === 'undefined') {
+    if (
+      this.options.exit === true ||
+      typeof this.options.exit === 'undefined'
+    ) {
       this.options.exit = () => process.exit()
     }
 
@@ -217,9 +220,12 @@ export class Program extends (EventEmitter as new () => TypedEventEmitter<Events
            * From the yargs docs:
            * > Populated if any validation errors raised while parsing.
            * http://yargs.js.org/docs/#api-parseargs-context-parsecallback
+           * This seems to be incorrect though, and err is populated when any
+           * error is thrown inside the command handler.
            */
           if (err) {
-            console.error(err)
+            // Ignore err value, which encourages users to deliberately handle
+            // error conditions in their programs.
           }
 
           if (isPromise(argv.__promise)) {