Update compiler.ts Improve TypeScript typings (vercel#11074)

Improve TypeScript typings

packages/next/build/compiler.ts

@@ -1,50 +1,48 @@
-import webpack, { Stats } from 'webpack'
+import webpack, { Stats, Configuration } from 'webpack'
 
 export type CompilerResult = {
-  errors: Error[]
-  warnings: Error[]
+  errors: string[]
+  warnings: string[]
 }
 
 function generateStats(result: CompilerResult, stat: Stats): CompilerResult {
-  const { errors, warnings } = stat.toJson({
-    all: false,
-    warnings: true,
-    errors: true,
-  })
+  const { errors, warnings } = stat.toJson('errors-warnings')
   if (errors.length > 0) {
-    result.errors.push(...(errors as any))
+    result.errors.push(...errors)
   }
 
   if (warnings.length > 0) {
-    result.warnings.push(...(warnings as any))
+    result.warnings.push(...warnings)
   }
 
   return result
 }
 
 export function runCompiler(
-  config: webpack.Configuration | webpack.Configuration[]
+  config: Configuration | Configuration[]
 ): Promise<CompilerResult> {
   return new Promise(async (resolve, reject) => {
     const compiler = webpack(config)
-    compiler.run((err: Error, statsOrMultiStats: any) => {
-      if (err) {
-        return reject(err)
-      }
+    compiler.run(
+      (err: Error, statsOrMultiStats: { stats: Stats[] } | Stats) => {
+        if (err) {
+          return reject(err)
+        }
 
-      if (statsOrMultiStats.stats) {
-        const result: CompilerResult = statsOrMultiStats.stats.reduce(
-          generateStats,
-          { errors: [], warnings: [] }
+        if ('stats' in statsOrMultiStats) {
+          const result: CompilerResult = statsOrMultiStats.stats.reduce(
+            generateStats,
+            { errors: [], warnings: [] }
+          )
+          return resolve(result)
+        }
+
+        const result = generateStats(
+          { errors: [], warnings: [] },
+          statsOrMultiStats
         )
         return resolve(result)
       }
-
-      const result = generateStats(
-        { errors: [], warnings: [] },
-        statsOrMultiStats
-      )
-      return resolve(result)
-    })
+    )
   })
 }