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<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie-edge">
<title>Document</title>
<script>
var flag = true ;//flag布尔型 true代表1
var flag1 = false ;//flag1 布尔型 false代表0
console.log(flag + 1);//true参与运算当1来看
//如果一个变量声明未赋值就直接输出 就是 undefined 未定义数据类型
var str ;
console.log(str);
var variable = undefined ;
console.log(variable + 'pink');//undefinedpink
console.log(variable + 1 );//NaN undefined 和数字相加 最后的结果是 NaN
//null 空值
var space = null ;
console.log(space + 'pink');
console.log(space + 1 );//1 空值和数字相加 返回的是原来的值
</script>
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<body>
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Learn from station b [JavaScript basic grammar-dom-bom-js-es6 new grammar-jQuery-data visualization echarts black horse pink teacher front-end basic video tutorial (more than 500 episodes) continued-Bilibili] https://b23. tv/YGIdOrv