Element: append() メソッド

append(param1)
append(param1, param2)
append(param1, param2, /* … ,*/ paramN)

let div = document.createElement("div");
let p = document.createElement("p");
div.append(p);
console.log(div.childNodes); // NodeList [ <p> ]

let div = document.createElement("div");
div.append("Some text");
console.log(div.textContent); // "Some text"

let div = document.createElement("div");
let p = document.createElement("p");
div.append("Some text", p);
console.log(div.childNodes); // NodeList [ #text "Some text", <p> ]

let div = document.createElement("div");
with (div) {
  append("foo");
}
// ReferenceError: append is not defined