[localization-plugin] Add a feature for injecting custom localized values into a compilation

.vscode/launch.json

@@ -38,7 +38,9 @@
 				"--inspect-brk",
 				"${workspaceFolder}/apps/heft/lib/start.js",
 				"--debug",
-				"test-watch"
+				"test",
+				"--test-path-pattern",
+        "${fileBasenameNoExtension}"
 			],
 			"skipFiles": ["<node_internals>/**"],
 			"outFiles": [],

common/changes/@rushstack/webpack5-localization-plugin/main_2025-06-27-23-15.json

@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@rushstack/webpack5-localization-plugin",
+      "comment": "Add a feature for injecting custom localized values into a compilation via the `getCustomDataPlaceholderForValueFunction` function on an instance of the `LocalizationPlugin`.",
+      "type": "minor"
+    }
+  ],
+  "packageName": "@rushstack/webpack5-localization-plugin"
+}

common/reviews/api/webpack5-localization-plugin.api.md

@@ -14,6 +14,11 @@ import type { IPseudolocaleOptions } from '@rushstack/localization-utilities';
 import type { LoaderContext } from 'webpack';
 import type { WebpackPluginInstance } from 'webpack';
 
+// @internal (undocumented)
+export interface _ICustomDataPlaceholder extends IValuePlaceholderBase {
+    valueForLocaleFn: ValueForLocaleFn;
+}
+
 // @public (undocumented)
 export interface IDefaultLocaleOptions {
     fillMissingTranslationStrings?: boolean;
@@ -121,30 +126,40 @@ export interface IPseudolocalesOptions {
 export type IResolvedMissingTranslations = ReadonlyMap<string, ILocaleFileData>;
 
 // @public (undocumented)
-export interface _IStringPlaceholder {
+interface IStringPlaceholder extends IValuePlaceholderBase {
     locFilePath: string;
     stringName: string;
-    suffix: string;
     translations: ReadonlyMap<string, ReadonlyMap<string, string>>;
-    value: string;
 }
+export { IStringPlaceholder }
+export { IStringPlaceholder as _IStringPlaceholder }
 
 // @public (undocumented)
 export interface ITrueHashPluginOptions {
     hashFunction?: (contents: string | Buffer) => string;
     stageOverride?: number;
 }
 
+// @public (undocumented)
+export interface IValuePlaceholderBase {
+    suffix: string;
+    value: string;
+}
+
 // @public
 export class LocalizationPlugin implements WebpackPluginInstance {
     constructor(options: ILocalizationPluginOptions);
     // (undocumented)
     addDefaultLocFileAsync(context: LoaderContext<{}>, localizedFileKey: string, localizedResourceData: ILocalizationFile): Promise<Record<string, string>>;
     apply(compiler: Compiler): void;
     // @internal (undocumented)
-    getDataForSerialNumber(serialNumber: string): _IStringPlaceholder | undefined;
+    _getCustomDataForSerialNumber(suffix: string): _ICustomDataPlaceholder | undefined;
+    // @beta (undocumented)
+    getCustomDataPlaceholderForValueFunction(valueForLocaleFn: ValueForLocaleFn, placeholderUniqueId: string): string;
     // (undocumented)
-    getPlaceholder(localizedFileKey: string, stringName: string): _IStringPlaceholder | undefined;
+    getPlaceholder(localizedFileKey: string, stringName: string): IStringPlaceholder | undefined;
+    // @internal (undocumented)
+    _getStringDataForSerialNumber(suffix: string): IStringPlaceholder | undefined;
     // @internal (undocumented)
     readonly _options: ILocalizationPluginOptions;
 }
@@ -156,6 +171,9 @@ export class TrueHashPlugin implements WebpackPluginInstance {
     apply(compiler: Compiler): void;
 }
 
+// @public (undocumented)
+export type ValueForLocaleFn = (locale: string, chunk: Chunk) => string;
+
 // (No @packageDocumentation comment for this package)
 
 ```

webpack/webpack5-localization-plugin/README.md

@@ -209,6 +209,17 @@ use the true hash of the content, rather than an intermediate hash that is share
 Note that this option is not compatible with the `runtimeLocaleExpression` option and will cause an error if
 both are set.
 
+## Custom Localized Data
+
+If you need to provide custom localized data, you can use the `getCustomDataPlaceholderForValueFunction` method
+of the plugin. This method takes a function that receives a locale name and the chunk
+that the placeholder is used in and returns a string that will be used as a placeholder for the localized data.
+The provided function will be called for each locale that is used in the build and the returned value will replace
+the returned placeholder in the output.
+
+Note that this may produce unexpected results if there are no other localized values in the chunk that the
+placeholder is used in.
+
 ## Links
 
 - [CHANGELOG.md](https://github.com/microsoft/rushstack/blob/main/webpack/localization-plugin/CHANGELOG.md) - Find