JavaScript中双符号的运算详解

var i = 5.1;
var j = 5.5;
console.log(~~i); // 5
console.log(~~j); // 5

~~-5.1 // 5
Math.floor(-5.1) // -6
~~-5.5 // 5
Math.floor(-5.5) // -6

Math.ceil(x)
Returns the smallest integer greater than or equal to a number.
Math.floor(x)
Returns the largest integer less than or equal to a number.
~~
是向0计算，取值向0靠拢

var a = 1;
var b = null;
var c = '';
var d = 'code';
console.log(!!a); // true
console.log(!!b); // false
console.log(!!c); // false
console.log(!!d); // true