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<!DOCTYPE html>
<html>
<body>
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<h1>JavaScript 运算符</h1>
<h2>?? 运算符</h2>
<p>如果第一个参数是 null 或 undefined，?? 运算符将返回第一个参数。否则返回第二个参数。</p>
​
<p id="demo"></p>
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<script>
let name = null;
let text = "missing";
let result = name ?? text;
document.getElementById("demo").innerHTML = "名字是 " + result; 
</script>
​
</body>
</html>
​