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
NumberStringBooleanFunctionObject-
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： NumberStringBooleanSymbol-
ObjectFunctionArrayDateRegExp
nullundefined
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
