JS中隐式转换面试问题总结

隐式转换

1.题目：

//1.
console.log(1 + "true");

console.log(1 + true);

console.log(1 + undefined);

console.log(1 + null);

//2.
console.log("2" > 10);

console.log("2" > "10");   

// 多个字符从左往右依次比较
console.log("abc" > "b");
console.log("abc" > "aad");
console.log(undefined == undefined);    
console.log(undefined == null);     
console.log(null == null);      
console.log(NaN == NaN);    

//3.
console.log([] == 0);     

console.log(![] == 0);     

console.log(![] == []);     

console.log([] == []);      

console.log({} == !{});     

console.log({} == {});  

2.相关知识

1. Number转换

   console.log(Number(null))	//0
   console.log(Number(""))		//0
   console.log(Number(undefined))	//NaN
   

2. Boolean转换

   let x = 0; //null undefined "" NaN false
   console.log(Boolean(x));
   //x所取六个值都转换为false
   

3.解答：

console.log(1 + "true");    //"1true"

console.log(1 + true);  // 2

console.log(1 + undefined);
// 1 + Number(undefined) = 1 + NaN = NaN
console.log(1 + null);
// 1 + Number(null) = 1 + 0 = 1

console.log("=============================");

console.log("2" > 10);
// Number("2") = 2 (2 > 10)     false
console.log("2" > "10"); //两个字符串比较 Unicode编码,true    

// 多个字符从左往右依次比较
console.log("abc" > "b");
// false
console.log("abc" > "aad");
// true
console.log(undefined == undefined);    //true
console.log(undefined == null);     //true
console.log(null == null);      //true
console.log(NaN == NaN);    //false

console.log("=============================");

console.log([] == 0);       // true
// [].valueOf().toString() = ""
// Number([].valueOf().toString()) = 0
console.log(![] == 0);      // true
// ![] 先逻辑Boolean转换，再转换Number
// Boolean([].valueOf().toString()) = false
// Number(false) = 0

console.log(![] == []);     // true
// 结合上面两题，Number(false) == Number("")
console.log([] == []);      //false 地址不同

console.log({} == !{});     // false
// {}.valueOf().toString() = [object Object]
// Number({}.valueOf().toString()) = NaN
// !{} ---> Boolean({}.valueOf().toString()) = true
// Number(true) = 1
console.log({} == {});  //false