feat(tools-react-native): Add mergeTransformerConfigs function to @rnx-kit/tools-react-native#4044
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feat(tools-react-native): Add mergeTransformerConfigs function to @rnx-kit/tools-react-native#4044
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tido64
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Mar 25, 2026
| * Type guard to check if a value is a plain object (i.e., a record). This is used to ensure that we only attempt to | ||
| * recursively merge plain objects in the `simpleObjectMerge` function. | ||
| */ | ||
| function isRecord(value: unknown): value is Record<string, unknown> { |
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Should this be parameterized?
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| function isRecord(value: unknown): value is Record<string, unknown> { | |
| function isRecord<K extends string | number = string, V = unknown>(value: unknown): value is Record<K, V> { |
| function simpleObjectMerge( | ||
| ...options: Record<string, unknown>[] | ||
| ): Record<string, unknown> { | ||
| /** @type {Record<string, unknown>} */ |
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Stray JSDoc
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| /** @type {Record<string, unknown>} */ |
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| const getTransformOptionsFns = configs | ||
| .map((config) => config?.getTransformOptions) | ||
| .filter((fn) => typeof fn === "function"); |
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Whenever you see .map followed by .filter, please rewrite it as .reduce or a for-loop instead:
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| const getTransformOptionsFns = configs | |
| .map((config) => config?.getTransformOptions) | |
| .filter((fn) => typeof fn === "function"); | |
| const getTransformOptionsFns = configs | |
| .reduce<TransformerConfigT["getTransformOptions"][]>((result, config) => { | |
| const getTransformOptions = config?.getTransformOptions; | |
| if (typeof getTransformOptions === "function") { | |
| result.push(getTransformOptions); | |
| } | |
| return result; | |
| }, []); |
JavaScript's implementation is eager and will cause additional unnecessary allocations.
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| equal(result.minifierPath, "/minifier-b"); | ||
| equal(result.hermesParser, false); | ||
| equal(result.enableBabelRuntime, true); |
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Can we use deepEqual to ensure that nothing else gets added?
| { hermesParser: false }, | ||
| { hermesParser: true } | ||
| ); | ||
| equal(result.hermesParser, true); |
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Can we use deepEqual to ensure that nothing else gets added?
| transform: { | ||
| experimentalImportSupport: true, | ||
| inlineRequires: false, | ||
| nonInlinedRequires: ["react"], |
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Can you add a test for how arrays should be handled?
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Description
This adds a utility for merging transformer configurations to tools-react-native to help with transformer merging. I added this as part of a new submodule called metro-utils for general metro utility functions.
I considered various other places for this. The main package it would make sense in is @rnx-kit/metro-config but that isn't a utility package used by other packages in react-native and has issues if you try to include it as a dependency of the cli. Given the already existing utilities for resolving packages from metro it made the most sense to add it to tools-react-native.
The function itself is straightforward and will perform a flat merge (which is what metro does) except if there are multiple getTransformOptions implementations in which case they will be wrapped with the results merged together in order of precedence. This function is used to initialize the babel transformers, it is called once with the results being serialized via json to the metro transformers.
Having the merge function be baked into the CLI is required so that any transform options being set by a transformer configuration don't get stomped by the esbuild transformer which is always added at the end. The esbuild transform options will take precedence still, but things like custom options can still be used in this case.