JAVAScript的數據類型在大的方向上分為兩類：1）primitive types and 2）object tyeps。
其一 primitive types包括常規的 numbers，string， booleans 以及特殊類型的 null 和 undefined。而且以上五類都是immutuable types；
其二，object types 包括object，以及特殊類型的object即array。其他比如 Set，Map，typed array, RegExp and Date types.
一、Numbers
Numeric literal 表示 十六進制，二進制和八進制：
//integer literals
> 0xff
255
> 0b1011
11
> 0o377
255
> 377
377
//floating-point literals
undefined
> 6.02e23
6.02e+23
> 1.47e-23
1.47e-23
//Arithmetic
Math.hypo
//Infinity
Infinity // A positive number too big to represent
Number.POSITIVE_INFINITY // Same value
1/0 // => Infinity
Number.MAX_VALUE * 2 // => Infinity; overflow
-Infinity // A negative number too big to represent
Number.NEGATIVE_INFINITY // The same value
-1/0 // => -Infinity
-Number.MAX_VALUE * 2 // => -Infinity
NaN // The not-a-number value
Number.NaN // The same value, written another way
0/0 // => NaN
Infinity/Infinity // => NaN
Number.MIN_VALUE/2 // => 0: underflow
-Number.MIN_VALUE/2 // => -0: negative zero
-1/Infinity // -> -0: also negative 0
-0
// The following Number properties are defined in ES6
Number.parseInt() // Same as the global parseInt() function
Number.parseFloat() // Same as the global parseFloat() function
Number.isNaN(x) // Is x the NaN value?
Number.isFinite(x) // Is x a number and finite?
Number.isInteger(x) // Is x an integer?
Number.isSafeInteger(x) // Is x an integer -(2**53) < x < 2**53?
Number.MIN_SAFE_INTEGER // => -(2**53 - 1)
Number.MAX_SAFE_INTEGER // => 2**53 - 1
Number.EPSILON // => 2**-52: smallest difference between numbers
// 浮點數
18,437,736,874,454,810,627 只有這么多浮點數，能被表示出來。
// rouding problems
//BigInt
//Date and time
let timestamp = Date.now(); // The current time as a timestamp (a number).
let now = new Date(); // The current time as a Date object.
let ms = now.getTime(); // Convert to a millisecond timestamp.
let iso = now.toISOString(); // Convert to a string in standard format.
二、String and Text
// 1.string literals
// 2.escape sequences
// 3.string methods
// 4.template literals (tagged template literals)
// 5.Pattern Matching
/[1-9][0-9]*/;
三、Boolean Values
只有 true 和 false 這兩項。
四、null and undefined
> typeof(null)
'object'
五、Symbols
let s = Symbol.for("shared");
let t = Symbol.for("shared");
s === t // => true
s.toString() // => "Symbol(shared)"
Symbol.keyFor(t) // => "shared"
六、Global
- Global constants like undefined, Infinity, and NaN
- Global functions like isNaN(), parseInt() (§3.9.2), and eval() (§4.12)
- Constructor functions like Date(), RegExp(), String(), Object(), and Array() (§3.9.2)
- Global objects like Math and JSON (§6.8)
七、Immutable Primitives and Mutable Object Referece
> function equalArray(a, b) {
... if (a === b) return true;
... if (a.length !== b.length) return false;
... for (let i = 0; i < a.length; i++) {
..... if (a[i] !== b[i]) return false;
..... }
... return true;
... }
八、Type Conversions
implicite conversion and explicite conversions
九、Variable Declaration and Assignment
// Destructuring Assignment
[x,y] = [x+1,y+1]; // Same as x = x + 1, y = y + 1
[x,y] = [y,x]; // Swap the value of the two variables
// Same as const sin=Math.sin, cos=Math.cos, tan=Math.tan
const {sin, cos, tan} = Math;
//此處與Python的用法完全一致。
