转换为布尔值

代表空、否定的值会转换为false ， 如 * 、0 、 NaN、null、undefined

其余的值会被转换为true

    console.log(Boolean("")); //false
    console.log(Boolean(0)); //false
    console.log(Boolean(NaN)); //false
    console.log(Boolean("null")); //false
    console.log(Boolean(undefined)); //false
    console.log(Boolean("小白")); //true
    console.log(Boolean(12)); //true