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));
