js强转

Posted 2020-12-25 栲霓杞蛙

js有六大数据类型:String,Number,Boolean,Null,Undefined,Object。其中前五项数据类型被称作原始类型。

typeof 操作符返回值有"string","number","boolean","undefined","object","function"。其中Null类型的返回值是"object",而Function作为Object类型的子类，返回的结果却是"function"

非数值类型转换为数值类型规则

//空字符串被转换为0
console.log(Number(""));//0

//字符串只包含数字,返回十进制数
console.log(Number("19"));//19

//字符串是十六进制形式，返回相等的十进制
console.log(Number("0x19"));//25

//字符串是浮点形式，返回浮点数
console.log(Number("12.5"));//12.5

//字符串是浮点形式，返回浮点数
console.log(Number("1.234e2"));//123.4

//除以上的字符串形式都返回NaN
console.log(Number("a")); //NaN

//Boolean true 返回1
console.log(Number(true));//1

//Boolean false 返回0
console.log(Number(false));//0

//数字形式，返回原参数
console.log(Number(23));//23

//Null类型返回0
console.log(Number(null));//0

//Undefined类型返回NaN
console.log(Number(undefined));//NaN         

 对象类型转换有特殊的规则

 将对象类型强转为Number类型时，先调用对象的valueOf()方法,如果方法的返回值是原始类型，那么就Number(valueOf的返回值),如果方法的返回类型是Object类型，那么会进一步调用对象的toString()方法。toString()方法的返回值如果是原始类型，则Number(toString的返回值)，如果toString()方法的返回值是Object类型，那么最终报错了

<script>
    var obj = {
        name:"pmx",
        age:20,
        valueOf:function(){
            console.log("value of");
            return this.name;
        },
        toString:function(){
            console.log(\'to string\');
            return this.name;
        }
    }
</script>

Number(\'pmx\')的结果为NaN

 

<script>
var obj = {
    name:"pmx",
    age:20,
    valueOf:function(){
        console.log("value of");
        return null
    },
    toString:function(){
        console.log(\'to string\');
        return this.name;
    }
}
</script>

Number(null)的结果是0

 

<script>
var obj = {
    name:"pmx",
    age:20,
    valueOf:function(){
        console.log("value of");
        return {}
    },
    toString:function(){
        console.log(\'to string\');
        return this.name;
    }
}
</script>

valueOf()返回的是非原始类型，那么调用obj的toString()方法，Number(\'pmx\')结果为NaN

 

<script>
var obj = {
    name:"pmx",
    age:20,
    valueOf:function(){
        console.log("value of");
        return {}
    },
    toString:function(){
        console.log(\'to string\');
        return {};
    }
}
</script>

toString()的返回结果是Object类型，最后Number强制转换失败，报错了

 

<script>
var obj = {
    name:"pmx",
    age:20,
    valueOf:function(){
        console.log("value of");
        return {}
    },
    toString:function(){
        console.log(\'to string\');
        return {
            a:10,
            valueOf:function(){
                return 10;
            },
            toString:function(){
                return \'a\'
            }
        };
    }
}
</script>

toString()方法返回的对象中自定义了valueOf()方法和toString()方法，仍然报错，说明数值强转不会递归进行