I don't understand this 1 line in the svelte code

numbers=numbers

<script>
    let numbers = [1, 2, 3, 4];

    function addNumber() {
        // numbers = [...numbers, numbers.length+1];
        //numbers[numbers.length] = numbers.length+1
            numbers.push(numbers.length + 1);
            numbers = numbers;    
    }

    $: sum = numbers.reduce((t, n) => t + n, 0);
</script>

<p>{numbers.join(' + ')} = {sum}</p>

<button on:click={addNumber}>
    Add a number
</button>