How does JavaScript determine whether it is a number?

Methods for JavaScript to determine whether it is a number: 1. Use the [isNaN()] function; 2. Use regular expressions; 3. Use the return value of [parseFloat()].

The operating environment of this tutorial: Windows 7 system, JavaScript version 1.8.5, DELL G3 computer.

JavaScript method to determine whether it is a number:

Method 1: Use isNaN() function

isNaN() function It is a global function that comes with js. The isNaN() function is used to check whether its parameter is a non-numeric value.

If value x is a special non-numeric value NaN (or can be converted to such a value), the returned value is true; if value x is another value, false is returned.

The disadvantage of isNaN() is that null, space and empty string will be treated as 0

NaN: Not a Number

<script>
document.write(isNaN(123));  //数字      ----false
document.write(isNaN(-1.23));  //数字    ----false
document.write(isNaN(5-2));  //数字      ----false
document.write(isNaN(0));  //数字        ----false
document.write(isNaN("Hello"));  //字符串 ----true
document.write(isNaN("2005/12/12"));  //字符串----true
</script>

Disadvantage: isNaN() will Null, spaces and empty strings are treated as 0, so the check is not strict.

So process it and use it with the typeof operator.

Example:

// true:数值型的，false：非数值型
function myIsNaN(value) {
   return (typeof value === 'number' && !isNaN(value));
}
myIsNaN(10);      ----true
myIsNaN(null);    ----false
myIsNaN( );      ----false
myIsNaN();       ----false

Note: If it is '36.3', typeof value === 'number', it will return false. If you want string type numbers to also return true, you can use the following Method 3

Method 2: Use regular expression

(1) to check as long as it is a number (including positive and negative integers, 0 and positive and negative floating point numbers) It returns 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 = /^[0-9]+.?[0-9]*/; //判断是否是数字。
  
    if(regPos.test(val) ){
        return true;
    }else{
        return false;
    }
}

(2). If the positive and negative numbers are verified, it returns 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;
    } 
}

Method 3: Use the return value of parseFloat()

parseFloat() function parses a string and returns a floating point number.

This function specifies whether the first character in the string is a number. If it is, the string is parsed until it reaches the end of the number, and the number is returned as a number rather than as a string.

Usage: Parse the string specified in the parameter into a floating point number and return it.

/**
* 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) cannot judge an empty string or a space

If it is an empty string, space or null, and isNaN is processed as a number 0,

And parseInt and parseFloat return an error message, which is caused by the loose isNaN check.

Related free learning recommendations: javascript video tutorial

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