Why won't this object method return a boolean value Javascript

Question

My app's js file includes this bit here:

var drawer = document.getElementById('b_001');

drawer.isOpen = function() {
  this.classList.contains('open');
};

When I call it in the console, drawer.isOpen() , I expect a boolean value, true or false . However, undefined is returned instead. Why is this?

Answer 1

你需要一个退货声明

 return this.classList.contains('open');

Answer 2

You'll have to return it:

drawer.isOpen = function() {
  return this.classList.contains('open');
//^ here  
};

If a function doesn't return anything, the return value is considered undefined , as this snippet demonstrates:

 var report = document.querySelector('#result'); report.innerHTML += doStuff(5); // nothing returned report.innerHTML += '<br>'+addFive(5); // a result is returned function doStuff(val) { val = val || 0; val += 5; } function addFive(val) { val = val || 0; val += 5; return val; } 

 <div id="result"></div>