/* 가져오기 */
const text = someNode.textContent;

/* 설정하기 */
someOtherNode.textContent = string;
let doc = document;
console.log(doc.textContent); // null

let doctype = document.doctype;
console.log(doctype.textContent); // null
<p id="a">환영합니다!</p>
const ele = document.getElementById("a");
console.log(ele.textContent); // "환영합니다!"
<p id="a"></p>
const ele = document.getElementById("a");
console.log(ele.textContent); // ""
<div id="container"><p>반갑습니다!</p></div>
const container = document.getElementById("container");
console.log(container.textContent); // "반갑습니다!"
<p id="a"><strong>중요:</strong> 환영합니다!</p>
<button type="button" id="btn">텍스트 내용을 설정</button>
const ele = document.getElementById("a");
const btn = document.getElementById("btn");

btn.addEventListener("click", () => {
    ele.textContent = "반갑습니다!";
});
실제 적용된 모습 '버튼'을 실제로 클릭해 보세요!
<div id="example">
    Hello,    world!
    This is a test.
</div>
<script>
    let text = document.getElementById("example").textContent;
    console.log(text);
    /* 출력:
            Hello,    world!
            This is a test.   
    */
</script>
let element = document.getElementById('example');
element.textContent = `
    Hello,    world!
    This is a test.
`; // 공백과 줄바꿈 포함