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<!DOCTYPE html>
<html>
<body>
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<h1>JavaScript 字符串</h1>
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<h2>substr() 方法</h2>
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<p>substr() 提取字符串的一部分：</p>
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<p id="demo"></p>
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<script>
let text = "Hello world!"; 
let result = text.substr(0, 1);
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document.getElementById("demo").innerHTML = result;
</script>
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</body>
</html>
​