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**opposite sets of each of these metacharacters by using their upper case letters**
\ escape char after
\d 0-9 
\D represents any non-digit character
\w alphanumeric letters and digits
\W any non-alphanumeric character (such as punctuation)
\b which matches the boundary between a word and a non-word character. 
\. match single char (letter/digit/whitespace), wildcard
[] match any single char within [] will pass
[^] not to match any within [], excludes specific char

ruichuang

before install rvm, install the mpapis public key.
$brew install gnupg gnupg2
$gpg --keyserver hkp://keys.gnupg.net
     --recv-keys 409B6B1796C275462A1703113804BB82D39DC0E3
follow rvm website
test: rvm list / rvm use *.*.* / ruby -v

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The function object has a length property set to the number of formal parameters it is declared with.
In JavaScript, variables don't have types -- values have types. 
The typeof operator always returns a string
Variables that have no value = "undefined", an "undeclared" variable is one that has not been formally declared in the accessible scope.
The typeof operator returns "undefined" even for "undeclared" (or "not defined") variables


let a = 13.6 | 0;[ floor float numbers]
  All bitwise operations except unsigned right shift, >>>, work on signed 32-bit integers. So using bitwise operations will 
  convert a float to an integer. - slightly faster than Math.floor - 32-bit signed integers only

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npm view webpack versions ------ list all versions
npm show webpack version  ------ list latest version

webpack 1 vs 2
- 2 handles ex6 imports

run webpack
 ./node_modules/webpack/bin/webpack.js <input file> <output file>
 
 in package.json, do:

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git log --graph
git log --graph --decorate --oneline
git log --graph --full-history --all --pretty=format:"%h%x09%d%x20%s"

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The websocket communication relies on the client-server logic, where a persistent connection between a server 
and a client always exists.

the server opens a dedicated port where clients get connected. once happens, all connected app can send outgoing message to port and listen
to it for incoming message without additional requset.

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IEEE 802.11 Radiotap Capture header (radiotap)
       radiotap.antenna  Antenna
           Unsigned 32-bit integer
           Antenna number this frame was sent/received over (starting at 0)

       radiotap.channel  Channel
           Unsigned 32-bit integer
           802.11 channel number that this frame was sent/received on

       radiotap.channel.freq  Channel frequency

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If this json file won't become too big over the time you should try:

Create a javascript object with the table array in it
var obj = {
   table: []
};
Add some data to it like
obj.table.push({id: 1, square:2});
Convert it from an object to string with stringify
var json = JSON.stringify(obj);

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scanning devices within wifi area

http://www.libelium.com/products/meshlium/smartphone-detection/

	•	The MAC address of the wireless interface, which allows to identify it uniquely.
	•	The strength of the signal (RSSI), which gives us the average distance of the device from the scanning point.
	•	The vendor of the smartphone (Apple, Samsung, etc)
	•	The WiFi Access Point where the user is connected (if any) and the Bluetooth friendly name. Users not connected to an AP will be showed as "free users".
	•	The Class of Device (CoD) in case of Bluetooth which allows us to differentiate the type of device (smartphone, handsfree, computer, LAN/network AP). With this parameter we can differentiate among pedestrians and vehicles.

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unary plus +
The unary plus operator precedes its operand and evaluates to its operand but attempts to convert it into a number

+3     // 3
+'3'   // 3
+true  // 1
+false // 0
+null  // 0
+function(val){  return val } // NaN