function w3_open() {
document.getElementById("mySidebar").style.display = "block";
document.getElementById("w3-button").style.display = "none"
document.getElementById("w3-buttondos").style.display ="inline-block"
}
function w3_close() {
document.getElementById("mySidebar").style.display = "none";
document.getElementById("w3-buttondos ").style.display = "none"
document.getElementById("w3-button").style.display ="inline-block"
}<button class="w3-button w3-teal" onclick="w3_open()">☰</button>
<button class="w3-buttondos w3-teal" onclick="w3_close()">✖</button>it says me -> Uncaught TypeError: document.getElementById(…) is null
>Solution :
document.getElementById does not target a node by its class="" but by its id=""
<button id="w3-button" class="w3-button w3-teal" onclick="w3_open()">☰</button>
To target an element by its class you can use document.querySelector
document.querySelector(".w3-button")