Question

下午好！

我找到了此代码，我很惊讶键盘上的"backspace"将其转换为NUMBER：

     let x = null;              
     x = prompt("Write something: ", ""); 
    if((x == null) || (x == ""))   {
    alert("Nothing was written");
    } else {
    if("NaN" == 1*x+"")   {
    alert("A string was written");
    }  else    {
    alert("A number was written");
    }
    }

为什么？而且-如何治疗呢？

然后，我的第二个问题：
上面写着let x = null。我可以改写let x = ""吗？还是只有let x？

非常感谢！

Answer 1

let x;
x = prompt("Write something: ", "");

console.log("NaN" == 1); //this will always be false
console.log(1 * x); //x is multiplied by 1 if it is a number
                    //if x is a letter prints 'NaN'
                    //if x is null prints 0 because x is converted to a number and when we do this conversion null becomes 0
                    //if x is empty space also prints zero because when javascript converts " " to a number the result is 0
console.log(x + ""); //x is converted to a string if it is a number, but this essentially just prints the value of x
console.log("NaN" == 1 * x + "");
//because of the order of operations, our first step is to multiply 1 by x
    //if x is a space, the result is 0 | 1 * " " = 0
    //if x is null, the result is 0 | 1 * null = 0
    //if x is a letter, returns NaN | 1 * A = NaN
    //if x is a number, returns the number | 1 * 2 = 2
//next we add "" to whatever value we got in the first step
    //if the current value of x is 0 we are left with "NaN" == 0, which is false
    //if the current value of x is a letter, we are left with "NaN" == "NaN", which is true
    //if the current value of x is a number, we are left with "NaN" == *a number* which is false


if ((x === null) || ( x.trim() === "")) { //check if x is empty space
    alert("Nothing was written");
} else {
    if (Number(x) === "NaN") { //will return true if x contains letters
        alert(Number(x));
    } else {
        alert(Number(x)); //we end up here if x is a number
    }
}

好的，以上内容可以回答您的第一个问题。至于第二个，初始化x并不重要，因为在下一行中给它分配了一个值。该值是一个字符串，一旦开始使用数学运算，JavaScript只会尝试将其转换为数字。

Answer 2

在JS中，“” == 0等于true，因此在尝试执行+“”时最终显示为+ 0

在Javascript中，为什么在我的代码中空格键会“转换”为数字？

2 个答案: