What is the 'global' object in NodeJS

问题:

I've just seen a weird behaviour of the this keyword in NodeJS environment. I'm listing them with code. I've run this codes with NodeJS v6.x, with a single JavaScript file.

While testing with one line of code as follows, whether with or without the 'use strict' statement, this points to an empty object {}.

console.log(this)

But, when I'm running the statement within a self executing function like,

(function(){
  console.log(this);
}());

It's printing a really big object. Seems to me the Global execution context object created by NodeJS environment.

And while executing the above function with a 'use strict' statement, expectedly it's printing undefined

(function(){
  'use strict';

  console.log(this);
}());

But, while working with browser (I've tested only with Chrome), the first three examples yield the window object and the last one gave undefined as expected.

The behaviour of the browser is quite understandable. But, in case of NodeJS, does it not create the execution context, until I'm wrapping inside a function?

So, most of the code in NodeJS runs with an empty global object?

回答1:

While in browsers the global scope is the window object, in nodeJS the global scope of a module is the module itself, so when you define a variable in the global scope of your nodeJS module, it will be local to this module.

You can read more about it in the NodeJS documentation where it says:

global

<Object> The global namespace object.

In browsers, the top-level scope is the global scope. That means that in browsers if you're in the global scope var something will define a global variable. In Node.js this is different. The top-level scope is not the global scope; var something inside an Node.js module will be local to that module.

And in your code when you write:

• console.log(this) in an empty js file(module) it will print an empty object {} referring to your empty module.
• console.log(this); inside a self invoking function, this will point to the global nodeJS scope object which contains all NodeJS common properties and methods such as require(), module, exports, console...
• console.log(this) with strict mode inside a self invoking function it will print undefined as a self invoked function doesn't have a default local scope object in Strict mode.

回答2:

Very interesting:

var JSON = require('circular-json');

console.log('1) ' + JSON.stringify(this, null, 2));

(function(){
    console.log('2) ' + JSON.stringify(this, null, 2));
}());

(function(){
  'use strict';
   console.log('3) ' + JSON.stringify(this, null, 2));
}());

will produce:

1) {}


2) {
  "global": "~",
  "process": {
    "title": "node",
    "version": "v6.9.1",
    "moduleLoadList": [
      "Binding contextify",
      "Binding natives",
      "NativeModule events",
      "NativeModule util",
      "Binding uv",
      "NativeModule buffer",
      "Binding buffer",
      "Binding util",
      "NativeModule internal/util",
      "NativeModule timers",
      "Binding timer_wrap",
      "NativeModule internal/linkedlist",
      "NativeModule assert",
      "NativeModule internal/process",
      "Binding config",
      "NativeModule internal/process/warning",
      "NativeModule internal/process/next_tick",
      "NativeModule internal/process/promises",                                                                                                              
      "NativeModule internal/process/stdio",                                                                                                                 
      "Binding constants",                                                                                                                                   
      "NativeModule path",                                                                                                                                   
      "NativeModule module",                                                                                                                                 
      "NativeModule internal/module",                                                                                                                        
      "NativeModule vm",                                                                                                                                     
      "NativeModule fs",                                                                                                                                     
      "Binding fs",                                                                                                                                          
      "NativeModule stream",                                                                                                                                 
      "NativeModule _stream_readable",                                                                                                                       
      "NativeModule internal/streams/BufferList",                                                                                                            
      "NativeModule _stream_writable",                                                                                                                       
      "NativeModule _stream_duplex",                                                                                                                         
      "NativeModule _stream_transform",                                                                                                                      
      "NativeModule _stream_passthrough",                                                                                                                    
      "Binding fs_event_wrap",                                                                                                                               
      "NativeModule console",                                                                                                                                
      "Binding tty_wrap",                                                                                                                                    
      "NativeModule tty",                                                                                                                                    
      "NativeModule net",                                                                                                                                    
      "NativeModule internal/net",                                                                                                                           
      "Binding cares_wrap",                                                                                                                                  
      "Binding tcp_wrap",                                                                                                                                    
      "Binding pipe_wrap",                                                                                                                                   
      "Binding stream_wrap",                                                                                                                                 
      "Binding signal_wrap"                                                                                                                                  
    ],                                                                                                                                                       
    "versions": {                                                                                                                                            
      "http_parser": "2.7.0",                                                                                                                                
      "node": "6.9.1",                                                                                                                                       
      "v8": "5.1.281.84",                                                                                                                                    
      "uv": "1.9.1",                                                                                                                                         
      "zlib": "1.2.8",                                                                                                                                       
      "ares": "1.10.1-DEV",                                                                                                                                  
      "icu": "57.1",                                                                                                                                         
      "modules": "48",                                                                                                                                       
      "openssl": "1.0.2j"                                                                                                                                    
    },                                                                                                                                                       
    "arch": "x64",                                                                                                                                           
    "platform": "linux",                                                                                                                                     
    "release": {                                                                                                                                             
      "name": "node",                                                                                                                                        
      "lts": "Boron",                                                                                                                                        
      "sourceUrl": "https://nodejs.org/download/release/v6.9.1/node-v6.9.1.tar.gz",
      "headersUrl": "https://nodejs.org/download/release/v6.9.1/node-v6.9.1-headers.tar.gz"
    },
    "argv": [
      "/usr/local/bin/node",
      "/home/froth/freelancer-projects/thistest.js"
    ],
    "execArgv": [],
    "env": {
      "NVM_DIR": "/home/froth/.nvm",
      "LD_LIBRARY_PATH": "/opt/opencascade/lib",
      "CSF_UnitsDefinition": "/opt/opencascade/src/UnitsAPI/Units.dat",
      "CSF_GraphicShr": "/opt/opencascade/lib/libTKOpenGl.so",
      "CSF_EXCEPTION_PROMPT": "1",
      "LANG": "de_DE.UTF-8",
      "PROFILEHOME": "",
      "DISPLAY": ":0",
      "SHELL_SESSION_ID": "09b6f0f3b1d94c5f8aba3f8022075677",
      "NODE_PATH": "/usr/lib/node_modules",
      "COLORTERM": "truecolor",
      "NVM_CD_FLAGS": "",
      "MOZ_PLUGIN_PATH": "/usr/lib/mozilla/plugins",
      "CSF_IGESDefaults": "/opt/opencascade/src/XSTEPResource",
      "CSF_XCAFDefaults": "/opt/opencascade/src/StdResource",
      "XDG_VTNR": "1",
      "PAM_KWALLET5_LOGIN": "/tmp/kwallet5_froth.socket",
      "CSF_STEPDefaults": "/opt/opencascade/src/XSTEPResource",
      "XDG_SESSION_ID": "c2",
      "CSF_XSMessage": "/opt/opencascade/src/XSMessage",
      "USER": "froth",
      "DESKTOP_SESSION": "/usr/share/xsessions/awesome",
      "GTK2_RC_FILES": "/home/froth/.gtkrc-2.0",
      "PWD": "/home/froth/freelancer-projects",
      "HOME": "/home/froth",
      "XDG_SESSION_TYPE": "x11",
      "CSF_PluginDefaults": "/opt/opencascade/src/StdResource",
      "XDG_DATA_DIRS": "/usr/local/share/:/usr/share/:/var/lib/snapd/desktop",
      "NVM_IOJS_ORG_MIRROR": "https://iojs.org/dist",
      "KONSOLE_DBUS_SESSION": "/Sessions/1",
      "XDG_SESSION_DESKTOP": "",
      "CSF_StandardDefaults": "/opt/opencascade/src/StdResource",
      "CSF_StandardLiteDefaults": "/opt/opencascade/src/StdResource",
      "MMGT_CLEAR": "1",
      "KONSOLE_DBUS_WINDOW": "/Windows/1",
      "CSF_UnitsLexicon": "/opt/opencascade/src/UnitsAPI/Lexi_Expr.dat",
      "GTK_MODULES": "canberra-gtk-module",
      "MAIL": "/var/spool/mail/froth",
      "NVM_RC_VERSION": "",
      "CSF_XmlOcafResource": "/opt/opencascade/src/XmlOcafResource",
      "TERM": "xterm-256color",
      "SHELL": "/bin/bash",
      "KONSOLE_DBUS_SERVICE": ":1.23",
      "XDG_SESSION_CLASS": "user",
      "XDG_SEAT_PATH": "/org/freedesktop/DisplayManager/Seat0",
      "XDG_CURRENT_DESKTOP": "",
      "QT_LINUX_ACCESSIBILITY_ALWAYS_ON": "1",
      "KONSOLE_PROFILE_NAME": "Shell",
      "CASROOT": "/opt/opencascade",
      "NVM_NODEJS_ORG_MIRROR": "https://nodejs.org/dist",
      "COLORFGBG": "15;0",
      "XDG_SEAT": "seat0",
      "SHLVL": "2",
      "LANGUAGE": "",
      "WINDOWID": "29360134",
      "LOGNAME": "froth",
      "DBUS_SESSION_BUS_ADDRESS": "unix:path=/run/user/1000/bus",
      "XDG_RUNTIME_DIR": "/run/user/1000",
      "CSF_MDTVTexturesDirectory": "/opt/opencascade/src/Textures",
      "XAUTHORITY": "/home/froth/.Xauthority",
      "XDG_SESSION_PATH": "/org/freedesktop/DisplayManager/Session1",
      "PATH": "/home/froth/.gem/ruby/2.3.0/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/snap/bin:/usr/lib/jvm/default/bin:/opt/opencascade/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl",
      "CSF_LANGUAGE": "us",
      "CSF_SHMessage": "/opt/opencascade/src/SHMessage",
      "OLDPWD": "/home/froth",
      "_": "/usr/local/bin/node"
    },
    "pid": 4658,
    "features": {
      "debug": false,
      "uv": true,
      "ipv6": true,
      "tls_npn": true,
      "tls_alpn": true,
      "tls_sni": true,
      "tls_ocsp": true,
      "tls": true
    },
    "_needImmediateCallback": false,
    "execPath": "/usr/local/bin/node",
    "debugPort": 5858,
    "_events": {
      "SIGWINCH": [
        null,
        null
      ]
    },
    "_eventsCount": 4,
    "domain": null,
    "_exiting": false,
    "config": {
      "target_defaults": {
        "cflags": [],
        "default_configuration": "Release",
        "defines": [],
        "include_dirs": [],
        "libraries": []
      },
      "variables": {
        "asan": 0,
        "debug_devtools": "node",
        "force_dynamic_crt": 0,
        "gas_version": "2.23",
        "host_arch": "x64",
        "icu_data_file": "icudt57l.dat",
        "icu_data_in": "../../deps/icu-small/source/data/in/icudt57l.dat",
        "icu_endianness": "l",
        "icu_gyp_path": "tools/icu/icu-generic.gyp",
        "icu_locales": "en,root",
        "icu_path": "deps/icu-small",
        "icu_small": true,
        "icu_ver_major": "57",
        "node_byteorder": "little",
        "node_enable_d8": false,
        "node_enable_v8_vtunejit": false,
        "node_install_npm": true,
        "node_module_version": 48,
        "node_no_browser_globals": false,
        "node_prefix": "/",
        "node_release_urlbase": "https://nodejs.org/download/release/",
        "node_shared": false,
        "node_shared_cares": false,
        "node_shared_http_parser": false,
        "node_shared_libuv": false,
        "node_shared_openssl": false,
        "node_shared_zlib": false,
        "node_tag": "",
        "node_use_bundled_v8": true,
        "node_use_dtrace": false,
        "node_use_etw": false,
        "node_use_lttng": false,
        "node_use_openssl": true,
        "node_use_perfctr": false,
        "node_use_v8_platform": true,
        "openssl_fips": "",
        "openssl_no_asm": 0,
        "shlib_suffix": "so.48",
        "target_arch": "x64",
        "uv_parent_path": "/deps/uv/",
        "uv_use_dtrace": false,
        "v8_enable_gdbjit": 0,
        "v8_enable_i18n_support": 1,
        "v8_inspector": true,
        "v8_no_strict_aliasing": 1,
        "v8_optimized_debug": 0,
        "v8_random_seed": 0,
        "v8_use_snapshot": true,
        "want_separate_host_toolset": 0
      }
    },
    "stdout": {
      "connecting": false,
      "_hadError": false,
      "_handle": {
        "bytesRead": 0,
        "_externalStream": {},
        "fd": 9,
        "writeQueueSize": 0,
        "owner": "~process~stdout"
      },
      "_parent": null,
      "_host": null,
      "_readableState": {
        "objectMode": false,
        "highWaterMark": 16384,
        "buffer": {
          "head": null,
          "tail": null,
          "length": 0
        },
        "length": 0,
        "pipes": null,
        "pipesCount": 0,
        "flowing": null,
        "ended": false,
        "endEmitted": false,
        "reading": false,
        "sync": true,
        "needReadable": false,
        "emittedReadable": false,
        "readableListening": false,
        "resumeScheduled": false,
        "defaultEncoding": "utf8",
        "ranOut": false,
        "awaitDrain": 0,
        "readingMore": false,
        "decoder": null,
        "encoding": null
      },
      "readable": false,
      "domain": null,
      "_events": {},
      "_eventsCount": 3,
      "_writableState": {
        "objectMode": false,
        "highWaterMark": 16384,
        "needDrain": false,
        "ending": false,
        "ended": false,
        "finished": false,
        "decodeStrings": false,
        "defaultEncoding": "utf8",
        "length": 0,
        "writing": false,
        "corked": 0,
        "sync": false,
        "bufferProcessing": false,
        "writecb": null,
        "writelen": 0,
        "bufferedRequest": null,
        "lastBufferedRequest": null,
        "pendingcb": 1,
        "prefinished": false,
        "errorEmitted": false,
        "bufferedRequestCount": 0,
        "corkedRequestsFree": {
          "next": null,
          "entry": null
        }
      },
      "writable": true,
      "allowHalfOpen": false,
      "destroyed": false,
      "_bytesDispatched": 6,
      "_sockname": null,
      "_writev": null,
      "_pendingData": null,
      "_pendingEncoding": "",
      "server": null,
      "_server": null,
      "columns": 84,
      "rows": 84,
      "_type": "tty",
      "fd": 1,
      "_isStdio": true
    },
    "stderr": {
      "connecting": false,
      "_hadError": false,
      "_handle": {
        "bytesRead": 0,
        "_externalStream": {},
        "fd": 11,
        "writeQueueSize": 0,
        "owner": "~process~stderr"
      },
      "_parent": null,
      "_host": null,
      "_readableState": {
        "objectMode": false,
        "highWaterMark": 16384,
        "buffer": {
          "head": null,
          "tail": null,
          "length": 0
        },
        "length": 0,
        "pipes": null,
        "pipesCount": 0,
        "flowing": null,
        "ended": false,
        "endEmitted": false,
        "reading": false,
        "sync": true,
        "needReadable": false,
        "emittedReadable": false,
        "readableListening": false,
        "resumeScheduled": false,
        "defaultEncoding": "utf8",
        "ranOut": false,
        "awaitDrain": 0,
        "readingMore": false,
        "decoder": null,
        "encoding": null
      },
      "readable": false,
      "domain": null,
      "_events": {},
      "_eventsCount": 3,
      "_writableState": {
        "objectMode": false,
        "highWaterMark": 16384,
        "needDrain": false,
        "ending": false,
        "ended": false,
        "finished": false,
        "decodeStrings": false,
        "defaultEncoding": "utf8",
        "length": 0,
        "writing": false,
        "corked": 0,
        "sync": true,
        "bufferProcessing": false,
        "writecb": null,
        "writelen": 0,
        "bufferedRequest": null,
        "lastBufferedRequest": null,
        "pendingcb": 0,
        "prefinished": false,
        "errorEmitted": false,
        "bufferedRequestCount": 0,
        "corkedRequestsFree": {
          "next": null,
          "entry": null
        }
      },
      "writable": true,
      "allowHalfOpen": false,
      "destroyed": false,
      "_bytesDispatched": 0,
      "_sockname": null,
      "_writev": null,
      "_pendingData": null,
      "_pendingEncoding": "",
      "server": null,
      "_server": null,
      "columns": 84,
      "rows": 84,
      "_type": "tty",
      "fd": 2,
      "_isStdio": true
    },
    "stdin": {
      "connecting": false,
      "_hadError": false,
      "_handle": {
        "bytesRead": 0,
        "_externalStream": {},
        "fd": 12,
        "writeQueueSize": 0,
        "owner": "~process~stdin",
        "reading": false
      },
      "_parent": null,
      "_host": null,
      "_readableState": {
        "objectMode": false,
        "highWaterMark": 0,
        "buffer": {
          "head": null,
          "tail": null,
          "length": 0
        },
        "length": 0,
        "pipes": null,
        "pipesCount": 0,
        "flowing": null,
        "ended": false,
        "endEmitted": false,
        "reading": false,
        "sync": false,
        "needReadable": true,
        "emittedReadable": false,
        "readableListening": false,
        "resumeScheduled": false,
        "defaultEncoding": "utf8",
        "ranOut": false,
        "awaitDrain": 0,
        "readingMore": false,
        "decoder": null,
        "encoding": null
      },
      "readable": true,
      "domain": null,
      "_events": {},
      "_eventsCount": 4,
      "_writableState": {
        "objectMode": false,
        "highWaterMark": 0,
        "needDrain": false,
        "ending": false,
        "ended": false,
        "finished": false,
        "decodeStrings": false,
        "defaultEncoding": "utf8",
        "length": 0,
        "writing": false,
        "corked": 0,
        "sync": true,
        "bufferProcessing": false,
        "writecb": null,
        "writelen": 0,
        "bufferedRequest": null,
        "lastBufferedRequest": null,
        "pendingcb": 0,
        "prefinished": false,
        "errorEmitted": false,
        "bufferedRequestCount": 0,
        "corkedRequestsFree": {
          "next": null,
          "entry": null
        }
      },
      "writable": false,
      "allowHalfOpen": false,
      "destroyed": false,
      "_bytesDispatched": 0,
      "_sockname": null,
      "_writev": null,
      "_pendingData": null,
      "_pendingEncoding": "",
      "server": null,
      "_server": null,
      "isRaw": false,
      "isTTY": true,
      "fd": 0
    },
    "argv0": "node",
    "mainModule": {
      "id": ".",
      "exports": {},
      "parent": null,
      "filename": "/home/froth/freelancer-projects/thistest.js",
      "loaded": false,
      "children": [
        {
          "id": "/home/froth/freelancer-projects/node_modules/circular-json/build/circular-json.node.js",
          "exports": {},
          "parent": "~process~mainModule",
          "filename": "/home/froth/freelancer-projects/node_modules/circular-json/build/circular-json.node.js",
          "loaded": true,
          "children": [],
          "paths": [
            "/home/froth/freelancer-projects/node_modules/circular-json/build/node_modules",
            "/home/froth/freelancer-projects/node_modules/circular-json/node_modules",
            "/home/froth/freelancer-projects/node_modules",
            "/home/froth/node_modules",
            "/home/node_modules",
            "/node_modules"
          ]
        }
      ],
      "paths": [
        "/home/froth/freelancer-projects/node_modules",
        "/home/froth/node_modules",
        "/home/node_modules",
        "/node_modules"
      ]
    }
  },
  "console": {}
}


3) undefined

In 3) this is undefined as it is not autoboxed to an object in strict mode. That means that there is no root object in this context. If you do not use strict mode then your code is boxed by a parent scope.As you can see in the output, within nodejs there is lot of information about node internal stuff.

In 1) the output is an empty object because in the top-level code in a node module, this is the equivalent to module.exports and module.exports is empty in this example.

回答3:

Value of this in a node module:

this in NodeJS global scope is the current module.exports object, not the global object. This is different from a browser where the global scope is the global window object. Consider the following code executed in Node:

console.log(this);    // logs {}

module.exports.foo = 5;

console.log(this);   // log { foo:5 }

First we log an emty object because there are no values in module.exports in this module. Then we put foo on the module.exports object, when we then again log this we can see that it now logs the updated module.exports object.

How can we access the global object:

We can access the global object in node using the global keyword:

console.log(global);

The global object exposes a variety of useful properties about the environment. Also this is the place where function as setImmediate and clearTimeout are located.

回答4:

To start with documentation on Global context in node environment

In browsers, the top-level scope is the global scope. That means that in browsers if you're in the global scope var something will define a global variable. In Node.js this is different. The top-level scope is not the global scope; var something inside an Node.js module will be local to that module.

Each JS file is treated as a module. Node automatically wraps the code of a JS file in a self IIFE with exports, require, module, __filename, __dirname as parameters to the function.

Below is the screenshot of execution context using node-debug

If you run the below code, prints true which means this refers to exports in node.js. Best explained in this answer.

console.log(this === exports);

Which means at execution the code is wrapped something similar as below in Node.js separating your code from global context using a wrapper function context.

  var context = (function (exports, require, module, __filename, __dirname) {
       console.log(this) //This is my code
  });

  var module = {exports:{}};
   context.apply(module.exports, [module.exports, require, module, "FILE_NAME", "DIR_NAME"]);

---

Answer to the next point refer this documentation completely:

A function's this keyword behaves a little differently in JavaScript compared to other languages. It also has some differences between strict mode and non-strict mode.

so when you execute this code

(function(){
  console.log(this);
}());

prints the global object and in use strict mode prints undefined

---

Remember:

In Browser the function is not wrapped by IIFE/wrapper function context as in node, its directly executed on window object. Hence the calling context varies for Node.js and Browser.

Also read this.