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Javascript

2019-06-20 15:36:01   0  举报

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js自学

JavaScript

作者其他创作

大纲/内容

Have you ever read any part of the JS specification?

基础

值

Number

运算

浮点数运算

大数运算

精确计算

科学计数法问题

String

宿主环境下

V8引擎

单线程

单调用栈

异步过程

EventLoop

原理

执行过程

引擎接收代码

函数调用栈优先执行

其中可以发起异步事件

发起的异步事件要等函数调用站执行完毕

引入概念 asynchronous callback

调用栈stack中使用 web api xhr,setTimeout, 在Web API 中执行，执行完毕之后将callback 放入 task quene

子主题

测试

简单的eventLoop

资料

Philip Roberts Help I'm stuck in an event loop

异步

JavaScript 引擎本身也可以发起任务了：promise

宿主发起的任务

宏观任务

JS引擎发起的

微观任务

JavaScript 引擎等待宿主环境分配宏观任务

宏观任务下也可以发起promise, js必须保证其在一个宏观任务中完成

每个宏观任务又包含了一个微观任务队列

Types

Primitive Types

有哪些

undefined

string

number

boolean

object

symbol

各种类型判断

哪些场景需要类型判断？

typeof 不靠谱

其他的方案

Abstract Operation

coercion

equality

Typescript flow

Scope System

Object Basic

closure

closure is when a function 'remembers' the variables around it even when that function is executed elsewhere

keep access alive avoiding grabage collecting

函数式

摘抄

Confidence

the core principles of functional programming

make a function a fp function

函数有输入输出

输入参数

判断输入的参数个数

However, I suggest that arguments.length, and only that, is OK to keep using for those cases where you need to care about the passed number of arguments. A future version of JS might possibly add a feature that offers the ability to determine the number of arguments passed without consulting arguments.length; if that happens, then we can fully drop usage of arguments!

剩余扩展符处理超出个数的

具名参数不需要管参数的输入顺序了

输出

return 也是 flow control的一种

拥有多return的函数也就拥有了多个输出出口

Rather than returning from the function early, we used normal flow control (if logic) to determine the retValue's assignment. At the end, we simply return retValue.

I'm not unconditionally saying that you should always have a single return, or that you should never do early returns, but I do think you should be careful about the flow control part of return creating more implicitness in your function definitions. Try to figure out the most explicit way to express the logic; that will often be the best way.

no implicit output

use return to make explicit output

容易产生副作用

JavaScript uses references and reference-copies for arrays, objects, and functions, so we may create an accidental output from our function all too easily.

匿名函数

不便于debug

可读性也差

箭头函数 lexically anonymous

I cannot abide by the loss of naming. It's harder to read, harder to debug, and impossible to self-reference.

lets us use simple function expressions much like we'd use lazy expressions -- another favorite of the FPer.

Declarative Style

declarative code communicates more effectively than imperative code

we should be striving for declarative, self-explanatory code

Name every single function

In my practice, if I don't have a good name to use for a function, I name it TODO initially. I'm certain that I'll at least catch that later when I search for "TODO" comments before committing code.

管理输入

for stylistic purposes of readability, sometimes you'll want to define functions in a way that hides their inputs entirely!

多参数转单参数

例子

["1","2","3"].map( parseInt );

辅助函数1

辅助函数2

(v)=>(v)

例子

例子2 在转化函数中作为默认函数

var constant =
v =>
() =>
v;

some now some later