JavaScript从零开始 | 01

此篇笔记为从零开始学习JavaScript的第一篇。作为我系统性学习JavaScript的开始。此系列将会以英文为主。主要学习资料为Mozilla的Documentation。

JavaScript Building Blocks: Types

• Number
• String
• Boolean
• Function
• Object
• Symbol (new in ES2015，也就是ES6)
  除了以上基础Types之外，JavaScript同时支持使用null和undefined, 以及Array，Date和RegExp。因此，一种更正确的Types分类形式为：
• Number
• String
• Boolean
• Symbol
• Object
  • Function
  • Array
  • Date
  • RegExp
• null
• undefined

Numbers

JavaScript是不支持纯所谓Integer的，也就是说Number Type就是floating point。JavaScript使用IEEE754 double precision (64-bits) floating point。
JavaScript支持一个built-in的Object为Math，使用者可以用其进行一些数学运算。

Math.sin(3.5);
const circumference = 2 * Math.PI * r;

JS支持使用parseInt来将String转换为Number。Function takes an optional second argument as the base of the string representation:

parseInt('123', 10); // 123
parseInt('010', 10); // 10
parseInt('11', 2); // 3

There is a function called parseFloat which behaves similarly with parseInt but doesn't take a second argument. Instead, it always assumes base 10 representations.
An unary + sign can be used to convert string into numbers as well:

+ '42'; // 42
+ `010`; // 10
+ `0x10`; // 16

A special value called NaN (which stands for "Not a Number") is used in JS:

parseInt('foo', 10); // NaN

Any mathematical operation operating on NaN will also return NaN (which is quite toxic...):

NaN + 5; // NaN

One can use the built-in function isNaN to test whether a value is NaN or not:

isNaN(NaN); // true

JS also supports two Infinity values:

1 / 0; Infinity
-1 / 0; -Infinity

One can also test whether a value is infinity of not by using the isFinite function:

isFinite(1 / 0); // false
isFinite(-Infinity); // false
isFinite(NaN); // false

One thing to point out for comparing the difference between + and parseInt,parseFloat is that, two parse functions will terminate once they hit invalid characters and return the value up to that point, whereas the + operand will simply return NaN if a invalid argument is given:

+ '10.2abc'; // NaN
parseInt('10.2abc', 10); // 10
parseFloat('10.2abc'); // 10.2