focus()

当元素获得焦点时， focus 事件就会触发。

与 focus 相反的事件是 blur

HTML

<p id="log">Click on this document to give it focus.</p>

Css

.paused {
  background: #ddd;
  color: #555;
}

JavaScript

function pause() {
  document.body.classList.add("paused");
  log.textContent = "FOCUS LOST!";
}

function play() {
  document.body.classList.remove("paused");
  log.textContent =
    "This document has focus. Click outside the document to lose focus.";
}

const log = document.getElementById("log");

window.addEventListener("blur", pause);
window.addEventListener("focus", play);

参考

• js使原元素获取焦点以及移除焦点的方法