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import { mergeWith } from 'lodash';
/**
* Merge handler for lodash.mergeWith
*
* Note that a return value of `undefined`
* indicates to lodash that it should use
* its normal merge algorithm.
*
* In this case, we want to merge keys if
* they don't exists but when they do, we
* prefer to concatenate lists instead of
* overwriting them.
* @param {?Array<Function>} left existing handlers
* @param {Array<Function>} right new handlers to add
* @returns {Array<Function>} combined handlers
*/
const concatHandlers = ( left, right ) =>
Array.isArray( left ) ? left.concat( right ) : undefined;
export const mergeHandlers = ( ...handlers ) =>
handlers.length > 1
? mergeWith( Object.create( null ), ...handlers, concatHandlers )
: handlers[ 0 ];
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