非数值类型转换为数值类型

显式转换：特指Number(value)
隐式转换：非显式，比如“/”除号

1 let str="fly";
2 let n=Number(str);
3 alert("6"/"2");  //3

undefined:NaN
null:0
true:1; false:0
string: 纯数字字符串返回数字；空串返回0；非数值型字符返回error

console.log(Number("123"));
console.log(Number(""));
console.log(Number("fly"));
console.log(Number(true));
console.log(Number(false));
console.log(Number(undefined));
console.log(Number(null));