Add helpful error messages when a plugin cannot be found

src/command.ts

@@ -185,9 +185,13 @@ export default abstract class Command {
     }
   }
 
+  protected _helpPlugin() {
+    return getHelpPluginPackage(this.config.pjson)
+  }
+
   protected _help() {
-    const pluginPackage = getHelpPluginPackage(this.config.pjson)
-    const HHelp = require(pluginPackage).default
+    const helpPlugin = this._helpPlugin()
+    const HHelp = require(helpPlugin).default
     const help: HelpBase = new HHelp(this.config)
     const cmd = Config.Command.toCached(this.ctor as any as Config.Command.Class)
     if (!cmd.id) cmd.id = ''

src/main.ts

@@ -2,7 +2,6 @@ import * as Config from '@oclif/config'
 import {HelpBase} from '@oclif/plugin-help'
 
 import {Command} from '.'
-import {getHelpPluginPackage} from './util'
 
 export class Main extends Command {
   static run(argv = process.argv.slice(2), options?: Config.LoadOptions) {
@@ -37,8 +36,8 @@ export class Main extends Command {
   }
 
   protected _help() {
-    const pluginPackage = getHelpPluginPackage(this.config.pjson)
-    const HHelp = require(pluginPackage).default
+    const helpPlugin = this._helpPlugin()
+    const HHelp = require(helpPlugin).default
     const help: HelpBase = new HHelp(this.config)
     help.showHelp(this.argv)
     return this.exit(0)

src/util.ts

@@ -34,16 +34,21 @@ export function sortBy<T>(arr: T[], fn: (i: T) => sort.Types | sort.Types[]): T[
   return arr.sort((a, b) => compare(fn(a), fn(b)))
 }
 
-export function getHelpPluginPackage(pjson: Config['pjson']): string {
-  const configuredHelpPlugin = pjson && pjson.oclif && pjson.oclif.helpPlugin
-  const defaultHelpPlugin = '@oclif/plugin-help'
+export function getHelpPluginPackage(pjson: Config['pjson'], defaultPlugin = '@oclif/plugin-help'): string {
+  const configuredPlugin = pjson.oclif.helpPlugin
 
-  if (configuredHelpPlugin) {
+  if (configuredPlugin) {
     try {
-      require(configuredHelpPlugin)
-      return configuredHelpPlugin
+      require(configuredPlugin)
+      return configuredPlugin
     } catch {}
   }
 
-  return defaultHelpPlugin
+  try {
+    require(defaultPlugin)
+    return defaultPlugin
+  } catch {}
+
+  if (configuredPlugin) throw new Error(`Unable to load configured help plugin "${configuredPlugin}" from package.json`)
+  throw new Error('Could not load a help plugin, consider installing the @oclif/plugin-help package')
 }

test/command.test.ts

@@ -3,6 +3,7 @@ import {expect, fancy} from 'fancy-test'
 
 import Base, {flags} from '../src'
 import {TestHelpPluginConfig} from './helpers/test-help-plugin'
+import {getHelpPluginPackage} from '../src/util'
 
 // const pjson = require('../package.json')
 
@@ -269,6 +270,48 @@ USAGE
 `)
     })
 
+    fancy
+    .stdout()
+    .add('config', async () => {
+      const config: TestHelpPluginConfig = await Config.load()
+      config.pjson.oclif.helpPlugin = 'plugin-does-not-exist'
+      return config
+    })
+    .do(async ({config}) => {
+      class CMD extends Command {
+        config = config
+
+        _helpPlugin() {
+          return getHelpPluginPackage(this.config.pjson, '')
+        }
+      }
+
+      await CMD.run(['-h'])
+    })
+    .catch((error: Error) => expect(error.message).to.equal('Unable to load configured help plugin "plugin-does-not-exist" from package.json'))
+    .it('shows useful error message when configured help plugin cannot be loaded')
+
+    fancy
+    .stdout()
+    .add('config', async () => {
+      const config: TestHelpPluginConfig = await Config.load()
+      config.pjson.oclif.helpPlugin = undefined
+      return config
+    })
+    .do(async ({config}) => {
+      class CMD extends Command {
+        config = config
+
+        _helpPlugin() {
+          return getHelpPluginPackage(this.config.pjson, '')
+        }
+      }
+
+      await CMD.run(['-h'])
+    })
+    .catch((error: Error) => expect(error.message).to.equal('Could not load a help plugin, consider installing the @oclif/plugin-help package'))
+    .it('shows useful error message when no help plugin has been configured and the default cannot be loaded')
+
     describe('from a help plugin', () => {
       fancy
       .stdout()