请问为什么防抖函数返回的匿名函数的this指向input？为什么捕获不到event？

Question

    <input id="content" type="text">
    <script>
        let input = document.getElementById('content');
        let fn = debounce(searchResult, 500);
        input.addEventListener('input', fn, false);
        
        function searchResult(event) {
            let target = event.target; // Cannot read property 'target' of undefined
        }

        function debounce(fn, interval) {
            let timer = null;
            return function() {
                console.log(this); // <input id="content" type="text">
                if (timer) {
                    clearTimeout(timer);
                } 
                timer = setTimeout(function(){
                    fn();
                }, interval)
            }
        }

Answer 1

方法没写对，一般都是 arguments 然后 apply 进去

function debounce(fn, interval) {
            let timer = null;
            return function(event) {
                console.log(this); // <input id="content" type="text">
                if (timer) {
                    clearTimeout(timer);
                } 
                timer = setTimeout(function(){
                    fn(event);
                }, interval)
            }
        }

Answer 2

fn(this)

没给 fn 传参，肯定是拿不到咯