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export default function resolveExtensions(configuredExtensions, providers = []) {
// Combine all extensions possible for testing. Remove duplicate extensions.
const duplicates = new Set();
const seen = new Set();
const normalize = extensions => Array.isArray(extensions) ? extensions : Object.keys(extensions);
const combine = extensions => {
for (const ext of normalize(extensions)) {
if (seen.has(ext)) {
duplicates.add(ext);
} else {
seen.add(ext);
}
}
};
if (configuredExtensions !== undefined) {
combine(configuredExtensions);
}
for (const {main} of providers) {
combine(main.extensions);
}
if (duplicates.size > 0) {
throw new Error(`Unexpected duplicate extensions in options: ’${[...duplicates].join('’, ’')}’.`);
}
// Unless the default was used by providers, as long as the extensions aren't explicitly set, set the default.
if (configuredExtensions === undefined) {
if (!seen.has('cjs')) {
seen.add('cjs');
}
if (!seen.has('mjs')) {
seen.add('mjs');
}
if (!seen.has('js')) {
seen.add('js');
}
}
return [...seen];
}