diff --git a/lib/manager/npm/update/dependency/index.ts b/lib/manager/npm/update/dependency/index.ts
index ed939783d971a3f6bac9e8810ce437b1a972385d..9c94b58d7fd35ffa236c4d7c551047f2b3b77931 100644
--- a/lib/manager/npm/update/dependency/index.ts
+++ b/lib/manager/npm/update/dependency/index.ts
@@ -1,8 +1,48 @@
 import { dequal } from 'dequal';
+import type { PackageJson } from 'type-fest';
 import { logger } from '../../../../logger';
 import { matchAt, replaceAt } from '../../../../util/string';
 import type { UpdateDependencyConfig } from '../../../types';
 
+function replaceAsString(
+  parsedContents: PackageJson,
+  fileContent: string,
+  depType: string,
+  depName: string,
+  oldVersion: string,
+  newValue: string
+): string | null {
+  // Update the file = this is what we want
+  // eslint-disable-next-line no-param-reassign
+  parsedContents[depType][depName] = newValue;
+  // Look for the old version number
+  const searchString = `"${oldVersion}"`;
+  const newString = `"${newValue}"`;
+  // Skip ahead to depType section
+  let searchIndex = fileContent.indexOf(`"${depType}"`) + depType.length;
+  logger.trace(`Starting search at index ${searchIndex}`);
+  // Iterate through the rest of the file
+  for (; searchIndex < fileContent.length; searchIndex += 1) {
+    // First check if we have a hit for the old version
+    if (matchAt(fileContent, searchIndex, searchString)) {
+      logger.trace(`Found match at index ${searchIndex}`);
+      // Now test if the result matches
+      const testContent = replaceAt(
+        fileContent,
+        searchIndex,
+        searchString,
+        newString
+      );
+      // Compare the parsed JSON structure of old and new
+      if (dequal(parsedContents, JSON.parse(testContent))) {
+        return testContent;
+      }
+    }
+  }
+  // istanbul ignore next
+  throw new Error();
+}
+
 export function updateDependency({
   fileContent,
   upgrade,
@@ -30,41 +70,21 @@ export function updateDependency({
   }
   logger.debug(`npm.updateDependency(): ${depType}.${depName} = ${newValue}`);
   try {
-    const parsedContents = JSON.parse(fileContent);
+    const parsedContents: PackageJson = JSON.parse(fileContent);
     // Save the old version
     const oldVersion: string = parsedContents[depType][depName];
     if (oldVersion === newValue) {
       logger.trace('Version is already updated');
       return fileContent;
     }
-    // Update the file = this is what we want
-    parsedContents[depType][depName] = newValue;
-    // Look for the old version number
-    const searchString = `"${oldVersion}"`;
-    const newString = `"${newValue}"`;
-    let newFileContent = null;
-    // Skip ahead to depType section
-    let searchIndex = fileContent.indexOf(`"${depType}"`) + depType.length;
-    logger.trace(`Starting search at index ${searchIndex}`);
-    // Iterate through the rest of the file
-    for (; searchIndex < fileContent.length; searchIndex += 1) {
-      // First check if we have a hit for the old version
-      if (matchAt(fileContent, searchIndex, searchString)) {
-        logger.trace(`Found match at index ${searchIndex}`);
-        // Now test if the result matches
-        const testContent = replaceAt(
-          fileContent,
-          searchIndex,
-          searchString,
-          newString
-        );
-        // Compare the parsed JSON structure of old and new
-        if (dequal(parsedContents, JSON.parse(testContent))) {
-          newFileContent = testContent;
-          break;
-        }
-      }
-    }
+    let newFileContent = replaceAsString(
+      parsedContents,
+      fileContent,
+      depType,
+      depName,
+      oldVersion,
+      newValue
+    );
     // istanbul ignore if
     if (!newFileContent) {
       logger.debug(
@@ -93,33 +113,14 @@ export function updateDependency({
             'Upgraded dependency exists in yarn resolutions but is different version'
           );
         }
-        // Look for the old version number
-        const oldResolution = `"${String(parsedContents.resolutions[depKey])}"`;
-        const newResolution = `"${newValue}"`;
-        // Update the file = this is what we want
-        parsedContents.resolutions[depKey] = newValue;
-        // Skip ahead to depType section
-        searchIndex = newFileContent.indexOf(`"resolutions"`);
-        logger.trace(`Starting search at index ${searchIndex}`);
-        // Iterate through the rest of the file
-        for (; searchIndex < newFileContent.length; searchIndex += 1) {
-          // First check if we have a hit for the old version
-          if (matchAt(newFileContent, searchIndex, oldResolution)) {
-            logger.trace(`Found match at index ${searchIndex}`);
-            // Now test if the result matches
-            const testContent = replaceAt(
-              newFileContent,
-              searchIndex,
-              oldResolution,
-              newResolution
-            );
-            // Compare the parsed JSON structure of old and new
-            if (dequal(parsedContents, JSON.parse(testContent))) {
-              newFileContent = testContent;
-              break;
-            }
-          }
-        }
+        newFileContent = replaceAsString(
+          parsedContents,
+          newFileContent,
+          'resolutions',
+          depKey,
+          parsedContents.resolutions[depKey],
+          newValue
+        );
       }
     }
     return newFileContent;