@graphql-mesh/transport-rest/esm/directives/flatten.js

export function processFlattenAnnotations(field) {
    if (!field.resolve || field.resolve.name === 'defaultFieldResolver') {
        const fieldName = field.name;
        field.resolve = function flattenDirectiveHandler(root) {
            let result = root[fieldName];
            if (!Array.isArray(root)) {
                result = [result];
            }
            return result.flat(Infinity);
        };
    }
}