diff --git a/packages/next/src/build/webpack/loaders/css-loader/src/utils.ts b/packages/next/src/build/webpack/loaders/css-loader/src/utils.ts
index 0c1455d985ae5..5bea07bcf7a2c 100644
--- a/packages/next/src/build/webpack/loaders/css-loader/src/utils.ts
+++ b/packages/next/src/build/webpack/loaders/css-loader/src/utils.ts
@@ -216,7 +216,7 @@ function normalizeSourceMap(map: any, resourcePath: string) {
     // We should normalize path because previous loaders like `sass-loader` using backslash when generate source map
     newMap.sources = newMap.sources.map((source: string) => {
       // Non-standard syntax from `postcss`
-      if (source.indexOf('<') === 0) {
+      if (source.startsWith('<')) {
         return source
       }
 
@@ -297,7 +297,7 @@ function normalizeSourceMapForRuntime(map: any, loaderContext: any) {
 
     resultMap.sources = resultMap.sources.map((source: string) => {
       // Non-standard syntax from `postcss`
-      if (source.indexOf('<') === 0) {
+      if (source.startsWith('<')) {
         return source
       }
 
diff --git a/packages/next/src/build/webpack/loaders/postcss-loader/src/utils.ts b/packages/next/src/build/webpack/loaders/postcss-loader/src/utils.ts
index ea09bd5ef085f..5362598199abf 100644
--- a/packages/next/src/build/webpack/loaders/postcss-loader/src/utils.ts
+++ b/packages/next/src/build/webpack/loaders/postcss-loader/src/utils.ts
@@ -68,7 +68,7 @@ function normalizeSourceMapAfterPostcss(map: any, resourceContext: string) {
 
   // eslint-disable-next-line no-param-reassign
   newMap.sources = newMap.sources.map((source: string) => {
-    if (source.indexOf('<') === 0) {
+    if (source.startsWith('<')) {
       return source
     }
 
diff --git a/packages/next/src/client/components/react-dev-overlay/internal/helpers/format-webpack-messages.ts b/packages/next/src/client/components/react-dev-overlay/internal/helpers/format-webpack-messages.ts
index 280ecf41b82a2..a1b27a8d29499 100644
--- a/packages/next/src/client/components/react-dev-overlay/internal/helpers/format-webpack-messages.ts
+++ b/packages/next/src/client/components/react-dev-overlay/internal/helpers/format-webpack-messages.ts
@@ -118,7 +118,7 @@ function formatMessage(
   }
 
   // Cleans up verbose "module not found" messages for files and packages.
-  if (lines[1] && lines[1].indexOf('Module not found: ') === 0) {
+  if (lines[1] && lines[1].startsWith('Module not found: ')) {
     lines = [
       lines[0],
       lines[1]