Hi Eli,

From what I can see, error was triggered when printing out the final result string. I hope the refactoring makes it easier to debug the code step-by-step. 

Code:

function solve(array) {
    let register = {};
    array.forEach(line => {
        let [systemName, componentName, subcomponentName] = line.split(' | ');
        if (!register[systemName]) {
            register[systemName] = {};
            register[systemName][componentName] = [];
            register[systemName][componentName].push(subcomponentName);
        } else if (register[systemName] && !register[systemName][componentName]) {
            register[systemName][componentName] = [];
            register[systemName][componentName].push(subcomponentName);
        } else {
            register[systemName][componentName].push(subcomponentName);
        }
    })

    // Order the systemsName in register by amount of components, in descending order, as first criteria, and by alphabetical order as second criteria
    // as string [] only keys

    // let sortedSystemsName = Object.keys(register).sort((a, b) => {
    //     if (Object.keys(register[b]).length > Object.keys(register[a]).length) {
    //         return 1;
    //     } else if (Object.keys(register[b]).length === Object.keys(register[a]).length) {
    //         return a.localeCompare(b);
    //     } else {
    //         return -1;
    //     }
    // });


    sortedSystemsName = Object.entries(register).sort(([systemNameA, componentsA], [systemNameB, componentsB]) => {
        return Object.values(componentsB).length - Object.values(componentsA).length || systemNameA.localeCompare(systemNameB);
    });

    sortedSystemsName.forEach(([currentSystemName, components]) => {
        console.log(currentSystemName); // as string

        let componentCollection = Object.entries(components);
        // let sortedComponentsBySubcomponentsLength = Object.keys(register[currentSystemName]).sort((a, b) =>
        //     Object.keys((register[currentSystemName][b]).length - Object.keys((register[currentSystemName][a].length))));

        let sortedComponents = componentCollection
            .sort(([componentNameA, subcompenentsA], [componentNameB, subcompenentsB]) => subcompenentsB.length - subcompenentsA.length);

        // sortedComponentsBySubcomponentsLength.forEach((currentComponent => {
        //     console.log(`|||${currentComponent}`); // as string

        //     register[currentSystemName][currentComponent].forEach(subcomponent => console.log(`||||||${subcomponent}`));
        // }));

        sortedComponents.forEach((([conName, subCollection]) => {
            console.log(`|||${conName}`); // as string

            subCollection.forEach(sub => console.log(`||||||${sub}`));
        }));
    });
}