Number

• Binary: 0b10/0B10.
• Octal: 0o23/0O23.
• Hex: 0xFF.
• ** 指数运算符.
• BigInt.

const a = 2172141653
const b = 15346349309
const c1 = a * b
// => 33334444555566670000

const c2 = BigInt(a) * BigInt(b)
// => 33334444555566667777n

const inhabitantsOfLondon = 1_335_000
const distanceEarthSunInKm = 149_600_000

const fileSystemPermission = 0b111_111_000
const bytes = 0b1111_10101011_11110000_00001101
const words = 0xF3B_F00D

const massOfElectronInKg = 9.109_383_56e-31
const trillionInShortScale = 1e1_2

Number Conversion

x	Number(x)
undefined	NaN
null	0
boolean	false → 0, true → 1
number	x
bigint	-1n → -1, 1n → 1
string	'' → 0
	Other → parsed number, ignoring leading/trailing whitespace
symbol	Throws TypeError
object	Configurable ([Symbol.toPrimitive]()/valueOf())

assert.equal(Number(123.45), 123.45)
assert.equal(Number(''), 0)
assert.equal(Number('\n 123.45 \t'), 123.45)
assert.equal(Number('xyz'), Number.NaN)
assert.equal(Number(-123n), -123)
assert.equal(
  Number({
    valueOf() {
      return 123
    },
  }),
  123
)

Number Static Properties

• Number.NaN.
• Number.NEGATIVE_INFINITY.
• Number.POSITIVE_INFINITY.
• Number.MAX_SAFE_INTEGER.
• Number.MIN_SAFE_INTEGER.
• Number.EPSILON.
• Number.isNaN().
• Number.isFinite().
• Number.isInteger().
• Number.isSafeInteger().
• Number.toExponential().
• Number.toFixed().
• Number.toPrecision().
• Number.parseInt(string, radix).
• Number.parseFloat(string).

;(1234).toExponential()
// '1.234e+3'
;(1234).toExponential(5)
// '1.23400e+3'
;(1234).toExponential(1)
// '1.2e+3'
;(0.003).toExponential()
// '3e-3'
;(0.00000012).toFixed(10)
// '0.0000001200'
;(0.00000012).toFixed()
// '0'
;(10 ** 21).toFixed()
// '1e+21'
;(1234).toPrecision(3)
// '1.23e+3'
;(1234).toPrecision(4)
// '1234'
;(1234).toPrecision(5)
// '1234.0'
;(1.234).toPrecision(3)
// '1.23'

Not A Number

const numberType = typeof Number.NaN // 'number'

Number.isFinite(Number.NaN)
// false
Number.isNaN(Number.NaN)
// true
Number.isNaN(123)
// false
Number.isNaN('abc')
// false

function isNumber(value) {
  return typeof value === 'number' && Number.isFinite(value)
}

NaN

NaN === NaN -> false.

Infinity Number

Infinity represents all values greater than 1.7976931348623157e+308. Infinity will be converted to null with JSON.stringify().

const largeNumber = 1.7976931348623157e308

// eslint-disable-next-line no-loss-of-precision -- Infinity
const largerNumber = 1.7976931348623157e309

console.log(largeNumber) // 1.7976931348623157e+308
console.log(largerNumber) // Infinity
console.log(46 / 0) // Infinity
console.log(Number.POSITIVE_INFINITY) // Infinity
console.log(Number.MAX_VALUE) // Infinity

// eslint-disable-next-line no-loss-of-precision -- -Infinity
console.log(-1.7976931348623157e309) // -Infinity
console.log(-46 / 0) // -Infinity
console.log(Number.NEGATIVE_INFINITY) // -Infinity
console.log(Number.MIN_VALUE) // -Infinity

console.log(Math.max()) // -Infinity
console.log(Math.min()) // Infinity

Number.isFinite(Number.POSITIVE_INFINITY)
// false
Number.isFinite(Number.NEGATIVE_INFINITY)
// false
Number.isFinite(Number.NaN)
// false
Number.isFinite(123)
// true

Safe Number

• Safe integers:
  • Precision: 53 bits plus sign.
  • Range: (−2^53, 2^53).
• Array indices:
  • Precision: 32 bits, unsigned
  • Range: [0, 2^32−1).
  • Typed Arrays have a larger range of 53 bits (safe and unsigned).
• Bitwise operators:
  • Precision: 32 bits.
  • Range of unsigned right shift (>>>): unsigned, [0, 2^32).
  • Range of all other bitwise operators: signed, [−2^31, 2^31).

assert.equal(Number.MAX_SAFE_INTEGER, 2 ** 53 - 1)
assert.equal(Number.MIN_SAFE_INTEGER, -Number.MAX_SAFE_INTEGER)

assert.equal(Number.isSafeInteger(5), true)
assert.equal(Number.isSafeInteger('5'), false)
assert.equal(Number.isSafeInteger(5.1), false)
assert.equal(Number.isSafeInteger(Number.MAX_SAFE_INTEGER), true)
assert.equal(Number.isSafeInteger(Number.MAX_SAFE_INTEGER + 1), false)

Number.isInteger(-17)
// true
Number.isInteger(33)
// true
Number.isInteger(33.1)
// false
Number.isInteger('33')
// false
Number.isInteger(Number.NaN)
// false
Number.isInteger(Number.POSITIVE_INFINITY)
// false

Float Number

• 计算浮点数时, 应先计算整数, 再利用移位/乘法/除法转化为浮点数.
• 浮点值的精确度最高可达 17 位小数.

const a = (1 + 2) / 10 // a = 0.1 + 0.2;