I am not sure what I'm doing wrong. Letters are still being shown in the input.

<script>
    let value = "";
    
    function isNumber(value) {
        return !isNaN(value);
    }

    function handleInput(e) {
        let oldValue = value;
        let newValue = e.target.value;

        console.log(oldValue, newValue, "isNumber", isNumber(newValue));

        if (isNumber(newValue) && newValue.length < 17) {
            value = newValue;
        } else {
            value = oldValue;
        }
    }
</script>

<div class="container">
    <input
        {value}
        on:input|preventDefault={handleInput}
    />
</div>

Here's the REPL as well.