how to get the value from html slider in js

I’m coding an event function for my slider. I wanna get the value from the slider. However, it doesn’t work. And I found that the update function is never called, and _value is never updated.
Here is my HTML code

<input type="range" id="slider" min="20" max="200" value="20" step = "1">

Here is my js code

let slider = document.getElementById("slider");
let _value=20;
slider.addEventListener('oninput',update);
function update(event){
    console.log("hi");
    _value=slider.value;
}