Map() returns a new array, thus overwrites original structure of result transforming it from an object into an array !

Either avoid assigning result to receive the returning outcome of a map method, or just apply forEach on input.

The map() method creates a new array populated with the results of calling a provided function on every element in the calling array.

function solve(input) {
    let result = {};
    // input.map(x => { // also works
    input.forEach(x => {
        let [garage, carInfo] = x.split(' - ');

        if (!result.hasOwnProperty(garage)) {
            result[garage] = [];
        }

        result[garage].push(carInfo);
    });

    let sorted = Object.entries(result).sort((x, y) => x[0].localeCompare(y[0]));
    sorted.forEach(x => {
        console.log(`Garage № ${x[0]}`)
        x[1].forEach(x => console.log(`--- ${x}`))
    });
}