JSFiddle - React, Tailwind, and code Playground

HTML

<input type="checkbox" id="days_mon">Mon
<input type="checkbox" id="days_tue">Tue
<input type="checkbox" id="days_wed">Wed
<input type="checkbox" id="days_thr">thr
<input type="checkbox" id="days_fri">fri
<input type="checkbox" id="days_sat">sat
<input type="checkbox" id="days_sun">sun<br/>
<input type="button" id="check" value="check">

JavaScript

$(function(){
  
    $('#check').click(function(){
        if($("[id^='days_']:checked").length !=7 ){
        alert("Please select a day");}
        else{alert("works fine");}
    });
  
  });