笔记1

1. global.process常量

const {argv, argv0, execArgv, execPath} = process
//  argv 数组
argv.forEach(item => {
    console.log(item)
})
// 打印如下
/**
 * /usr/local/bin/node node启动的路径
 * /Users/nardo/Documents/study/node-study/process.js  执行node命令的脚本路径
 */
//  argv还可以有扩展的参数, 执行 node process.js --test a b=2
//  注意写在文件名之前的参数不会被打印
/**
 * /usr/local/bin/node node启动的路径
 * /Users/nardo/Documents/study/node-study/process.js  执行node命令的脚本路径
 * --test
 * a
 * b=2
 */
 // argv0 保存了argv第一个值的引用
console.log('argv0', argv0)
// 执行node --inspect process.js --test a b=2
console.log('execArgv', execArgv)  //  [ '--inspect' ]
// execPath
console.log('execPath', execPath)  //  /usr/local/bin/node  node启动的路径
// process.env
const { env } = process
//  process.cwd  打印出当前process的路径
console.log('cwd', process.cwd())   
// /Users/nardo/Documents/study/node-study

2. path模块

//  1. normalize    格式化路径
const {
    normalize
} = require('path')
console.log(normalize('/usr//local/bin')) //  /usr/local/bin
console.log(normalize('/usr/../local/bin')) //  /local/bin
//  2. join  拼接路径,调用时默认会调用normalize
const {
    join
} = require('path')
console.log(join('/usr', '../local', '/bin')) //  /usr/bin
//  3. resolve 将相对路径转为绝对路径
const {
    resolve
} = require('path')
console.log(resolve('./')) //  /Users/nardo/Documents/study/node-study
//  4. basename : 文件名， dirname: 路径不含文件名， extname: 拓展名
const {
    basename,
    dirname,
    extname
} = require('path')
const filePath = '/usr/bin/node/no.js'
console.log(basename(filePath)) //  no.js
console.log(dirname(filePath)) //  /usr/bin/node
console.log(extname(filePath)) //  .js
//  5. parse: 将路径解析成上述3个参数，format: 相反
const {
    parse,
    format
} = require('path')
let ret = parse(filePath)
console.log(ret)
/**
 * { root: '/',
  dir: '/usr/bin/node',
  base: 'no.js',
  ext: '.js',
  name: 'no' }
 */
// 注意： pathObject的优先级
// pathObject.dir存在时，忽略pathObject.root
// pathObject.base存在时，pathObject.ext和pathObject.name会被忽略
console.log(format({
    root: '/ignored',
    dir: '/bin/node/dir',
    base: 'yes.js'
})) 
//  /bin/node/dir/yes.js   root被忽略了
console.log(format({
    dir: '/bin/node/image',
    base: 'xmly.png',
    name: 'not_xmly',
    ext: 'jpg'
}))
//  /bin/node/image/xmly.png   ext和name被忽略了
/*  6. sep: 写path时的分隔符
* delimiter: 指PATH下的路径分隔符，macOS为 ':'  windows为 ';'
* win32: 
* posix:
*/
const {
    sep,
    delimiter,
    win32,
    posix
} = require('path')
console.log('sep:', sep)    //  /
console.log('win.sep', win32.sep)   //  \
console.log('PATH', process.env.PATH)   //  PATH /usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin
console.log('delimiter', delimiter) //  :
console.log('win.delimiter', win32.delimiter)   //  ;

3. paths

//  function (exports, require, module, __filename, __dirname) {}
//  1. __dirname  __filename
// const mod = require('./module_exports')
const path = require('path')
console.log('__dirname: ', __dirname) //    /Users/nardo/Documents/study/node-study
console.log('__filename: ', __filename) //    /Users/nardo/Documents/study/node-study/5_paths.js
console.log('process.cwd(): ', process.cwd()) //    /Users/nardo/Documents/study/node-study
console.log('./  ', path.resolve('./')) //    /Users/nardo/Documents/study/node-study
//  但如果此时不在当前目录执行脚本，而是返回上一层级
//  $ cd ..
//  打印结果是:
//  __dirname:  /Users/nardo/Documents/study/node-study
//  __filename:  /Users/nardo/Documents/study/node-study/5_paths.js
//  process.cwd():  /Users/nardo/Documents/study
//  ./   /Users/nardo/Documents/study
//  可以看到 process.cwd()和./变成了执行node命令的路径
/** 总结：
 * 1. __dirname, __filename总是返回文件的绝对路径
 * 2. process.cwd() 总是返回node执行所在文件目录
 * 3. './' 在require方法中总是相对当前文件所在文件夹，但在其他地方和process.cwd()一样，相对node启动文件夹目录
 */

4. global.Buffer

//  Buffer  全局变量
//  1. 用来处理二进制数据流
//  2. 实例类似整数数组，大小固定
//  3. C++代码在V8堆外分配物理内存
console.log(Buffer.alloc(10))   //  <Buffer 00 00 00 00 00 00 00 00 00 00>
console.log(Buffer.alloc(5, 1)) //  <Buffer 01 01 01 01 01>
console.log(Buffer.allocUnsafe(5, 1))   //  <Buffer 70 83 81 03 01>
console.log(Buffer.from([1, 2, 3])) //  <Buffer 01 02 03>
console.log(Buffer.from('test'))    //  <Buffer 74 65 73 74>
console.log(Buffer.from('test', 'base64'))    //    <Buffer b5 eb 2d>
//  Buffer.byteLength
console.log(Buffer.byteLength('test'))  // 4
console.log(Buffer.byteLength('测试'))    //  6
//  Buffer.isBuffer
console.log(Buffer.isBuffer({}))    //  false
console.log(Buffer.isBuffer(Buffer.from([1,2])))    //  true
//  Buffer.concat
const buf1 = Buffer.from('This ')
const buf2 = Buffer.from('is ')
const buf3 = Buffer.from('a  ')
const buf4 = Buffer.from('test ')
const buf5 = Buffer.from('! ')
const lastBuf = Buffer.concat([buf1, buf2, buf3, buf4, buf5])
console.log(lastBuf)    //  <Buffer 54 68 69 73 20 69 73 20 61 20 20 74 65 73 74 20 21 20>
//  buf.length
var buff = Buffer.from('This is a buffer')
console.log(buff.length) //  16  from方法调用时直接由内容决定buffer所占bytes
var buf = Buffer.alloc(10)
buf[0] = 2  //  该buffer占10bytes，但是只填充了1个
console.log(buf, buf.length)    //   <Buffer 02 00 00 00 00 00 00 00 00 00>  , 10
//  buf.toString()
console.log(buff.toString())    //  This is a buffer
console.log(buff.toString('base64'))    //  VGhpcyBpcyBhIGJ1ZmZlcg==