The standardized globalThis object translates this sentence into English.

The Birth Background of globalThis

The globalThis object introduced in ECMAScript 2019 (ES10) addressed the challenge of accessing the global object across different JavaScript environments. The global object varies by environment: window in browsers, global in Node.js, self in Web Workers, and this may be undefined in strict mode. This fragmentation necessitated complex environment detection:

// Traditional cross-environment global object access
const getGlobal = () => {
  if (typeof self !== 'undefined') return self
  if (typeof window !== 'undefined') return window
  if (typeof global !== 'undefined') return global
  throw new Error('Unable to locate global object')
}
const globalObj = getGlobal()

Core Features of globalThis

globalThis serves as a standardized way to access the global object, with the following key characteristics:

1. Environment Agnostic: Points to the top-level global object in any JavaScript runtime environment.
2. Non-configurable and Non-writable: Object.getOwnPropertyDescriptor(globalThis, 'globalThis') returns configurable: false.
3. Relationship with Existing Global Objects:

   // Browser environment
   globalThis === window // true
   
   // Node.js environment
   globalThis === global // true
   
   // Web Worker environment
   globalThis === self // true
   

Practical Use Cases

Cross-Environment Library Development

When developing libraries that need to run in both browser and Node.js environments, environment detection is no longer required:

// Unified global configuration
globalThis.LIB_CONFIG = {
  version: '1.0.0',
  debugMode: false
}

// Cross-environment access
function getConfig() {
  return globalThis.LIB_CONFIG || {}
}

Global Variable Management

Safely detect and manipulate global variables:

// Check if a global variable exists
if (!globalThis.Promise) {
  // Inject polyfill
  globalThis.Promise = require('es6-promise').Promise
}

// Safe pattern to avoid global pollution
(function(global) {
  const privateVar = 'Internal variable'
  global.publicApi = {
    getVar: () => privateVar
  }
})(globalThis)

Interaction with Module Systems

Access the true global object even in module scope:

// In ES modules
let globals = Object.keys(globalThis)
console.log(globals.includes('setTimeout')) // true

// Interoperability with CommonJS modules
if (typeof module !== 'undefined' && module.exports) {
  module.exports = { globalThis }
}

Compatibility and Transpilation

While modern environments generally support globalThis, legacy environment compatibility must be considered:

Polyfill Implementation

(function() {
  if (typeof globalThis === 'object') return
  Object.defineProperty(Object.prototype, '__magic__', {
    get: function() {
      return this
    },
    configurable: true
  })
  __magic__.globalThis = __magic__
  delete Object.prototype.__magic__
})()

Babel Configuration

Transform via @babel/plugin-transform-global-this plugin:

{
  "plugins": ["@babel/plugin-transform-global-this"]
}

TypeScript Support

Configure lib in tsconfig.json to include ES2019:

{
  "compilerOptions": {
    "lib": ["ES2019", "DOM"]
  }
}

Behavioral Differences in Special Environments

Different JavaScript engines exhibit subtle variations in globalThis implementation:

1. iframe Isolation in Browsers:

   // Main page
   globalThis.mainVar = 'value'
   
   // Inside iframe
   console.log(globalThis.mainVar) // undefined (cross-origin)
   

2. Node.js REPL Special Behavior:

   // In Node.js REPL
   globalThis === this // true (only in REPL)
   

3. Deno Environment:

   // In Deno
   globalThis === window // true (consistent with browsers)
   

Security Considerations and Best Practices

1. Avoid Direct Extension:

   // Not recommended
   globalThis.customApi = {...}
   
   // Recommended approach
   const GLOBAL_APIS = Symbol('global-apis')
   globalThis[GLOBAL_APIS] = {...}
   

2. Freeze Critical Global Properties:

   Object.defineProperty(globalThis, 'MY_APP', {
     value: Object.freeze({...}),
     writable: false,
     configurable: false
   })
   

3. Sandbox Environment Detection:

   function isSandboxed() {
     try {
       return globalThis === undefined || 
              globalThis.Function('return this')() !== globalThis
     } catch {
       return true
     }
   }
   

Performance Impact and Optimization

globalThis access performance across engines:

// Performance comparison test
console.time('window access')
for (let i = 0; i < 1e6; i++) window
console.timeEnd('window access')

console.time('globalThis access')
for (let i = 0; i < 1e6; i++) globalThis
console.timeEnd('globalThis access')

Typical results:

• Chrome: Nearly equivalent
• Firefox: globalThis ~5% slower
• Safari: Initial access ~15% slower, optimized in subsequent calls

Interaction with Other Language Features

Using with Proxy

Create a protective layer for the global object:

const guardedGlobal = new Proxy(globalThis, {
  set(target, prop, value) {
    if (prop.startsWith('_INTERNAL_')) {
      throw new Error(`Forbidden to set internal property ${prop}`)
    }
    return Reflect.set(target, prop, value)
  }
})

guardedGlobal.safeVar = 42 // Allowed
guardedGlobal._INTERNAL_temp = 'x' // Throws error

Integration with ShadowRealm API

const realm = new ShadowRealm()
realm.evaluate(`
  globalThis === this // true (in new global environment)
  globalThis === window // false
`)

Evolution of the Proposal

The standardization of globalThis went through several stages:

1. March 2018: Initial proposal (stage 1)
2. July 2018: Advanced to stage 2, discussed alternative names like System.global
3. January 2019: Finalized as globalThis and reached stage 3
4. June 2019: Released as part of ES2019

Key controversies:

• Naming (considered global, System.global, etc.)
• Web compatibility issues (some legacy browsers had non-overridable window)
• Handling conflicts with existing polyfills

Engine Implementation Details

Implementation approaches in major JavaScript engines:

1. V8 Engine:

   • Maintains global proxy in v8::Context
   • Accessed via internal %Global() function

2. SpiderMonkey:

   • Uses JS::GetGlobalForObject internal method
   • Special handling for Web Worker edge cases

3. JavaScriptCore:

   • Implemented as subclass of JSGlobalObject
   • Optimized access path in module scope

Common Misconceptions and Pitfalls

1. Incorrect Assumption of Configurability:

   // These operations will fail
   delete globalThis
   globalThis = null
   

2. Misunderstanding Module Scope:

   // Top-level this in modules is not globalThis
   console.log(this === globalThis) // false
   

3. Interaction with with Statement:

   const obj = { x: 1 }
   with(obj) {
     console.log(globalThis.x) // Still accesses global object
   }
   

Related Proposals and Future Directions

1. Standard Library Proposal:

   globalThis.std = {
     array: { ... },
     math: { ... }
   }
   

2. Global Registry Proposal:

   globalThis.registry = new FinalizationRegistry(...)
   

3. Cross-Realm Standardization: Discussions on unifying global access across iframes and Workers