<!DOCTYPE html>
<html>
<body>
<p>为函数参数设置默认值。</p>
<p id="demo"></p>
<script>
function myFunction(x, y) {
if (y === undefined) {
y = 0;
}
return x * y;
document.getElementById("demo").innerHTML = myFunction(4);
</script>
</body>
</html>
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