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FILE SPACING:

 # double space a file
 sed G

 # double space a file which already has blank lines in it. Output file
 # should contain no more than one blank line between lines of text.
 sed '/^$/d;G'

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function addQueryParameters (url, parameters) {
  const separator = /\?/.test(url) ? '&' : '?'
  const names = Object.keys(parameters)

  if (names.length === 0) {
    return url
  }

  return url + separator + names
    .map(name => {

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There are two kinds of statements:

1. Rulesets (or rules) that, as seen, associate a collection of CSS declarations to a condition described by a selector.
2. At-rules that start with an at sign, '@', followed by an identifier and then continuing up the end of the statement,


Rulesets are the main building blocks of a style sheet, which often consists of only a big list of them.
But there is other information that a Web author wants to convey in the style sheet, like the character set, 
other external style sheets to import, font face or list counter descriptions and many more.

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tmux cheat sheet

(C-x means ctrl+x, M-x means alt+x)

Prefix key

The default prefix is C-b. If you (or your muscle memory) prefer C-a, you need to add this to ~/.tmux.conf:

remap prefix to Control + a

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1. 为什么JavaScript是单线程？ ---为什么不是多线程呢
   JavaScript的单线程，与它的用途有关。作为浏览器脚本语言，JavaScript的主要用途是与用户互动， 
   以及操作DOM。这决定了它只能是单线程，否则会带来很复杂的同步问题。
   
   如果排队是因为计算量大，CPU忙不过来，倒也算了，但是很多时候CPU是闲着的，因为IO设备（输入输出设备）
   很慢（比如Ajax操作从网络读取数据），不得不等着结果出来，再往下执行。
   
   JavaScript语言的设计者意识到，这时主线程完全可以不管IO设备，挂起处于等待中的任务，先运行排在后面的任务。
   等到IO设备返回了结果，再回过头，把挂起的任务继续执行下去。
   

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1. 中间件接口
   {dispatch, getState} => next => action => your code
   
   中间件串联在一起时，在需要的时候通过调用next来调用下一个中间件，因此需要建立各个中间件next参数之间的连接(类似数据结构中链表节点中next);
   compose(...chain)(store.dispatch)语句相当于从后建立起了各个中间件next参数之间的连接关系，其中最后一个中间件的next的值为store.dispatch
   
   
   使用compose函数合成多个函数的时候，内层函数的返回值称为外层函数的参数，外层函数可以通过这个参数引用内层函数的的返回值
   
   

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action是一个对象，其中type属性是必须的，同时可以传入一些属性的数据。
action可以用actionCreactor进行创造,actionCreactor是一个函数,例如:
function addTodo(text) {
  return {
    type: ADD_TODO,
    text
  }
}

dispatch就是把action对象发送出去, 结合Action Creator发送动作的代码可以这样写

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1. Array.reduce
 
 有初始值：
 If an initialValue was provided in the call to reduce, then previousValue will be equal to initialValue 
 and currentValue will be equal to the first value in the array. 
 没有初始值：
 If no initialValue was provided, then previousValue will be equal to the first value in the array 
 and currentValue will be equal to the second
 
 一般数值的用法：

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###Arrow functions do not have this, arguments or other special names bound at all###
when the object is being created the name this is found in the enclosing scope.

name="YMJ"
var person = {
  name: "ymj",

  shout: () => console.log("my name is ", this.name)
}

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Babel compiles JSX down to React.createElement() calls.

const element = (                      const element = React.createElement(
  <h1 className="greeting">                     'h1',                        -----> element
    Hello, world!                                {className: 'greeting'},    -----> props of the element
  </h1>                                          'Hello, world!'             -----> children(include element and text node)
);                                     );


也就是说在jsx文件中类似<li>{number}</li>的不要将其看成是html结构而是对React.createElement的调用