现象

console.log(.1 + .2 === .3); // false

JS 进制互转

(10).toString(2); // '1010'
parseInt('1010', 2); // 10

整数十进制转二进制

除以 进制，取余，倒序

10 转 2

10 / 2 = 5, rem = 0
5 / 2 = 2, rem = 1
2 / 2 = 1, rem = 0
1 / 2 = 0, rem = 1

最终反着读: 1010

小数十进制转二进制

(0.1).toString(2)
'0.0001100110011001100110011001100110011001100110011001101'

乘以 进制，取整，正序

.1 * 2 = .2, rem = 0
.2 * 2 = .4, rem = 0
.4 * 2 = .8, rem = 0
.8 * 2 = .6, rem = 1
.6 * 2 = .2, rem = 1
.2 * 2 = .4, rem = 0
.4 * 2 = .8, rem = 0
.8 * 2 = .6, rem = 1
.6 * 2 = .2, rem = 1

最终：小数位不变 0. 后边是多少取多少: 0.0001100110011...

更多案例

.1 + .2
// 0.30000000000000004
2.3 + 2.4
// 4.699999999999999
1.0 - .9
// 0.09999999999999998
3 * .3
// 0.8999999999999999
1.21 / 1.1
// 1.0999999999999999

精度保留、四舍五入

Number.toPrecision

作用：不四舍五入，只保留精度（从第一个不为0的位数开始算精度）

let numObj = 5.123456

console.log(numObj.toPrecision())    // logs '5.123456'
console.log(numObj.toPrecision(5))   // logs '5.1235'
console.log(numObj.toPrecision(2))   // logs '5.1'
console.log(numObj.toPrecision(1))   // logs '5'
console.log(numObj.toPrecision(20))   // logs '5.1234560000000000102'
console.log(numObj.toPrecision(19))   // logs '5.123456000000000010'
console.log(numObj.toPrecision(18))   // logs '5.12345600000000001'
console.log(numObj.toPrecision(17))   // logs '5.1234560000000000'
console.log(numObj.toPrecision(16))   // logs '5.123456000000000'
console.log((0.2).toPrecision(3)); // 0.200

Number.toFixed

作用：四舍五入，保留小数点位数

let numObj = 12345.6789

console.log(numObj.toFixed()); // log 12346
console.log(numObj.toFixed(1)); // log 12345.7
console.log(numObj.toFixed(6)); // 12345.678900

// 特殊情况
numObj = 0.105;
console.log(numObj.toFixed()); // log 0
console.log(numObj.toFixed(1)); // log 0.1
console.log(numObj.toFixed(2)); // log 0.10 (这里精度有问题，没有进一)
console.log(numObj.toFixed(6)); // log 0.105000
console.log(numObj.toFixed(16)); // log 0.1050000000000000
console.log(numObj.toFixed(17)); // log 0.10500000000000000
console.log(numObj.toFixed(18)); // log 0.104999999999999996（出现精度问题了）