Function transform

src/utils/transform.ts:33–198

(
	source: SourceDescription,
	module: Module,
	pluginDriver: PluginDriver,
	options: NormalizedInputOptions
)

import { URL_TRANSFORM } from './urls';

export default async function transform(
	source: SourceDescription,
	module: Module,
	pluginDriver: PluginDriver,
	options: NormalizedInputOptions
): Promise<TransformModuleJSON> {
	const id = module.id;
	const sourcemapChain: DecodedSourceMapOrMissing[] = [];

	let originalSourcemap = source.map === null ? null : decodedSourcemap(source.map);
	const originalCode = source.code;
	let ast = source.ast;
	const transformDependencies: string[] = [];
	const emittedFiles: EmittedFile[] = [];
	let customTransformCache = false;
	const useCustomTransformCache = () => (customTransformCache = true);
	let pluginName = '';
	let currentSource = source.code;

	function transformReducer(
		this: PluginContext,
		previousCode: string,
		result: TransformResult,
		plugin: Plugin
	): string {
		let code: string;
		let map: string | ExistingRawSourceMap | { mappings: '' } | null | undefined;
		if (typeof result === 'string') {
			code = result;
		} else if (result && typeof result === 'object') {
			module.updateOptions(result);
			if (result.code == null) {
				if (result.map || result.ast) {
					options.onLog(LOGLEVEL_WARN, logNoTransformMapOrAstWithoutCode(plugin.name));
				}
				return previousCode;
			}
			if (result.attributes) {
				warnDeprecation(
					'Returning attributes from the "transform" hook is forbidden.',
					URL_TRANSFORM,
					false,
					options
				);
			}
			({ code, map, ast } = result);
		} else {
			return previousCode;
		}

		// strict null check allows 'null' maps to not be pushed to the chain,
		// while 'undefined' gets the missing map warning
		if (map !== null) {
			sourcemapChain.push(
				decodedSourcemap(typeof map === 'string' ? JSON.parse(map) : map) || {
					missing: true,
					plugin: plugin.name
				}