判断对象上是否存在指定key

'CSS' in window   //返回结果是true或者false

来看下面案例

检查pepole对象上是否存在名为css键

let pepole={css:function(){console.log('this is css')}};
'css' in pepole    // true

检查pepole对象上是否存在名为css1键

let pepole={css:function(){console.log('this is css')}};
'css1' in pepole    // false