contains()一つの要素にもう一つの要素が含まれているかどうかを判断します.

  parent
  
children

let parent = document.getElementById('parent')
let children = document.getElementById('children')
if (parent.contains(children)) {
  console.log(true)
}
if (children.contains(parent)) {
  console.log(false)
}

var obj = {name: 'myStric',age: 22};

if('name' in obj){
    console.log('yes');
} else {
    console.log('none');
}

//  “true”