How to determine whether a value is a number in js

JS method to determine whether a value is a number: 1. Use the [isNaN()] function, the code is [return typeof value === 'number' && !isNaN(value)]; 2. Use regular expressions ;3. Use the [parseFloat()] function.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, DELL G3 computer. This method is suitable for all brands of computers.

js method to determine whether a value is a number:

1. Disadvantages of using isNaN() function

isNaN() The reason is that null, spaces and empty strings will be treated as 0

NaN: Not a Number

/**
*判断是否是数字
*
**/
function isRealNum(val){
    // isNaN()函数 把空串 空格 以及NUll 按照0来处理 所以先去除，
    
　　if(val === "" || val ==null){
        return false;
　　}
   if(!isNaN(val)){　　　　
　　//对于空数组和只有一个数值成员的数组或全是数字组成的字符串，isNaN返回false，例如：'123'、[]、[2]、['123'],isNaN返回false,
   //所以如果不需要val包含这些特殊情况，则这个判断改写为if(!isNaN(val) && typeof val === 'number' )
　　　 return true; 
　　}
　else{ 
　　　　return false; 
　　} 
}

The most correct way for the isNaN() function to determine whether a value is a number is:

// true:数值型的，false：非数值型
  function myIsNaN(value) {
    return typeof value === 'number' && !isNaN(value);
  }

isNaN() detailed explanation

For empty arrays and arrays with only one numeric member, isNaN returns false.

• isNaN([]) // false

• isNaN([123]) // false

• isNaN(['123']) // false

The reason why the above code returns false is that these arrays can be converted into numerical values ​​by the Number function.

Therefore, before using isNaN, it is best to determine the data type.

function myIsNaN(value) {
  return typeof value === 'number' && !isNaN(value);
}

2. Use regular expressions　

(1). As long as the check is a number (including positive and negative integers, 0 and positive and negative floating point numbers), it will return true

/**
* 校验只要是数字（包含正负整数，0以及正负浮点数）就返回true
**/
function isNumber(val){
    var regPos = /^\d+(\.\d+)?$/; //非负浮点数
    var regNeg = /^(-(([0-9]+\.[0-9]*[1-9][0-9]*)|([0-9]*[1-9][0-9]*\.[0-9]+)|([0-9]*[1-9][0-9]*)))$/; //负浮点数
    if(regPos.test(val) && regNeg.test(val)){
        return true;
    }else{
        return false;
    }
}

(2). Verify the positive and negative numbers and return true

/**
* 校验正负正数就返回true
**/
function isIntNum(val){
    var regPos = / ^\d+$/; // 非负整数 
    var regNeg = /^\-[1-9][0-9]*$/; // 负整数
    if(regPos.test(val) && regNeg.test(val)){
        return true;
    }else{
        return false;
    } 
}

3. Use the parseFloat() function

/**
* 验证数据 是数字：返回true；不是数字：返回false
**/
function Number(val) {
　　if (parseFloat(val).toString() == "NaN") {
　　　　
　　　　return false;
　　} else {
　　　　return true;
　　}
}
//isNaN(val)不能判断空串或一个空格
//如果是一个空串、空格或null，而isNaN是做为数字0进行处理的，而parseInt与parseFloat是返回一个错误消息，这个isNaN检查不严密而导致的。

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