重写数字相关方法

isNaN


isNaN = function() {
    return Number(arguments[0]).toString()==="NaN"
    //注意，NaN===NaN  //false
}

console.log(isNaN(""))  //0
console.log(isNaN("3"))  //3
console.log(isNaN(0))  //0
console.log(isNaN(-0))  //-0
console.log(isNaN(null))  //0 注意
console.log(isNaN(false))  //0
console.log(isNaN(undefined))  //NaN
console.log(isNaN(NaN))  //NaN

console.log(isNaN("ss"))  //NaN
console.log(isNaN({}))  //NaN
console.log(isNaN({name:"ss"}))  //NaN
console.log(isNaN([]))  //0  注意
console.log(isNaN([0]))  //0  注意
console.log(isNaN([33]))  //33  注意
console.log(isNaN([1,2]))  //NaN


console.log(isNaN(new Number(2)) ) //2
console.log(isNaN(new String("2")) ) //2

parseFloat

// 总结：基础类型除了0，都返回NaN，对象也是，但数组先看第一项，在转

let num = 10;
parseFloat = function () {
    let type = Object.prototype.toString.call(arguments[0])
    if (type === "[object Number]") {
        return arguments[0]
    } else if (arguments[0]&&type === "[object String]") {
         // 1.正则
         let str = arguments[0].trim()
         let matchNum = str.match(/^-?\d+(\.\d+)?/g)?str.match(/^-?\d+(\.\d+)?/g)[0]:NaN
        return matchNum
       
        //2.字符类型判断
        // for (let i = 0; i < arguments[0].length; i++) {

        // }
    } else if (Array.isArray(arguments[0])) {
        return arguments.callee(arguments[0][0])
   } else {
        return NaN
    }
}

console.log(parseFloat("   12.777qewqewq11")) //12.777

parseInt

// 总结：基础类型除了0，都返回NaN，对象也是，但数组先看第一项，在转

let num = 10;
parseInt = function () {
    let type = Object.prototype.toString.call(arguments[0])
    if (type === "[object Number]") {
        return arguments[0]
    } else if (arguments[0]&&type === "[object String]") {
         // 1.正则
         let str = arguments[0].trim()
         let matchNum = str.match(/^-?\d+/g)?str.match(/^-?\d*/g)[0]:NaN
        return matchNum
       
        //2.字符类型判断
        // for (let i = 0; i < arguments[0].length; i++) {

        // }
    } else if (Array.isArray(arguments[0])) {
        return arguments.callee(arguments[0][0])
   } else {
        return NaN
    }
}

console.log(parseInt("   12qewq11")) //12
console.log(parseInt(""))  //NaN,注意
console.log(parseInt("0"))  //0,注意
console.log(parseInt(0))  //0
console.log(parseInt(-0))  //0,注意
console.log(parseInt(null))  //NaN
console.log(parseInt(undefined))  //NaN
console.log(parseInt(NaN))  //NaN
console.log(parseInt(false))  //NaN

console.log(parseInt("ss"))  //NaN
console.log(parseInt({}))  //NaN
console.log(parseInt({ name: "ss" }))  //NaN
console.log(parseInt([]))  //NaN
console.log(parseInt([0]))  //0，注意，将数组转为字符串，以逗号分隔
console.log(parseInt([1, 2]))  //1
console.log(parseInt(new Number(2)))  //2
console.log(parseInt("  23ewq"))  //23  //有左trim
console.log(parseInt(["  ", "", 1, 23, "4", "qew", 5]))  //NaN
console.log(parseInt(["", 1, 23, "4", "qew", 5]))  //NaN  
console.log(parseInt(["-8"]))  //-8
console.log(parseInt([0]))  //  0
console.log(parseInt([10]))  //10
console.log(parseInt([10, 12]))  //10
console.log(parseInt([-6, 12]))  //-6
console.log(parseInt([[["  12ewqewq"]]]))  //12

toPrecision

Number.prototype.toPrecision = function (length) {
    if (Object.prototype.toString.call(length) === "[object Number]") {
        if (this.toString().indexOf(".") > -1) {
            return Number(this.toString().slice(0, length + 1))
        } else {
            return Number(this.toString().slice(0, length))
        }
    } else {
        throw new Error("必须为数字")
    }
}

console.log(new Number(101.78458).toPrecision(new Number(5)))

toFixed

// 返回指定小数位数的表示形式

Number.prototype.toFixed = function(length) {
    if (Object.prototype.toString.call(length) === "[object Number]"&&!isNaN(length)) {

        let numStr = this.toString()
        let numLength = numStr.length
        let pointIndex = numStr.indexOf(".")

        if(pointIndex>-1){
            if(numLength-pointIndex-1>=length){
            return numStr.slice(0,pointIndex+1+length)
            }else{
                for(let i=0;i<length-numLength+pointIndex+1;i++){
                    numStr+="0"
                }
                return numStr
            }
        }else{
            numStr +="."
            for(let i=0;i<length;i++){
                numStr+="0"
            }
            return numStr
        }
    } else {
        throw new Error("必须为数字")
    }
}

let a = 12
let b = 12.1
let c = 12.1234567

console.log(a.toFixed(3))
console.log(b.toFixed(3))
console.log(c.toFixed(6))