如果没有自身改变，就会一直循环下去
n=0，只要n小于10就输出hello，和n的值，并且每次加1，直到n大于等于10,才结束
<!DOCTYPE html>
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<head><meta charset="UTF-8"><meta http-equiv="X-UA-Compatible" content="IE=edge"><meta name="viewport" content="width=device-width, initial-scale=1.0"><title>Document</title>
</head>
<body><script>var n = 0;while (n<10){console.log("hello",n)n++}</script>
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</html> 