• Skip to main content
  • Select language
  • Skip to search
MDN Web Docs
  • Technologies
    • HTML
    • CSS
    • JavaScript
    • Graphics
    • HTTP
    • APIs / DOM
    • WebExtensions
    • MathML
  • References & Guides
    • Learn web development
    • Tutorials
    • References
    • Developer Guides
    • Accessibility
    • Game development
    • ...more docs
Archive of obsolete content
  1. MDN
  2. Archive of obsolete content
  3. Archived open Web documentation
  4. Scope Cheatsheet

Scope Cheatsheet

In This Article
    1. var
    2. const
    3. let
    4. function
  1. Hoisting
  2. Parameters
  3. with captures assignments, but not var declarations
  4. eval may capture assignments, but not var declarations
  5. for heads
  6. catch variables are block-scoped
  7. let statements and expressions
  8. function oddities
  9. E4X selector predicates

JavaScript with Mozilla extensions has both function-scoped vars and block-scoped lets. Along with hoisting and dynamic behavior, scope in JavaScript is sometimes surprising.

Much covered here is not standard ECMAScript.

var

  • function-scoped
  • hoist to the top of its function
  • redeclarations of the same name in the same scope are no-ops

const

  • function-scoped
  • hoist to the top of its function
  • redeclarations of the same name in the same scope are rejected

let

  • block-scoped
  • hoist to the top of its block (not in ECMAScript 6!)
  • redeclarations illegal
  • behaves exactly the same as vars at function top-level (i.e. can be redeclared at function top-level even though cannot be elsewhere)

function

Three forms with different scope behavior:

  • declared: as a statement at the parent function top-level
    • behaves like a var binding that gets initialized to that function
    • initialization "hoists" to the very top of the parent function, above vars
  • statement: as a statement in a child block
    • behaves like a var binding that gets initialized to that function
    • does not hoist to the top of the parent function
  • expressed: inside an expression
    • bound in the expression only

Hoisting

Hoisting is perhaps the most surprising behavior and prone to the most hiccups. The general thing to remember is:

Every definition of a variable is really a declaration of the variable at the top of its scope and an assignment at the place where the definition is (see ECMAScript 6 exception below).

This figures into computation of upvars and shadowing as well.

Hoisting also cannot "cross paths", as a consequence of the coexistence of vars and lets. Doing so results in error.

  • lets cannot hoist above vars of the same name
function f() {
  {
    var x;
    let x; // error, hoisting crosses var x
  }
}
  • vars cannot hoist above lets of the same name
function f() {
  {
    let x;
    {
      var x; // error, hoisting crosses let x
    }
  }
}

Due to lets being vars at the function top-level, however, the following is okay.

function f() {
  let x;
  {
    var x; // okay, actually redeclaring a var, so acts as a no-op
  }
}

May interact surprisingly with catch-blocks.

function f() {
  try {
    throw "e";
  } catch(x) {
    var x;
    x = "catch"; // assignment to block-local x
  } 
  print(x); // undefined
}

In ECMAScript 6, let does not hoist the variable to the top of the block. If you reference a variable in a block before the let declaration for that variable is encountered, this results in a ReferenceError, because the variable is in a "temporal dead zone" from the start of the block until the declaration is processed.

function f() {
  console.log(x); // ReferenceError
  let x = 2;
}

Parameters

  • Multiple function parameters may share the same name. The last one is bound.
function f(x, x) {
  print(x);
}
f("foo", "bar"); // "bar"
  • Parameter names may shadow the function name itself inside the scope of the function.
function f(f) {
   print(f);
}
f("foo"); // "foo"
  • vars, however, do not shadow parameter names. And since function top-level lets are vars, lets don't either! The following prints "foo" because the declaration acts as a no-op as there is already a parameter named x.
function f(x, y) {
  var x;
  arguments[0] = "foo";
  print(x); // "foo"
}

with captures assignments, but not var declarations

Recall the hoisting rule above. Since with injects an object into the scope chain, what looks like assignments to variables might actually be assignments into properties of that object. And since variable definitions are actually two-part declaration and assignment, definitions of vars inside a with might not do what you think. The following two examples are equivalent.

function f() {
  var o = {x: "foo"};
  with (o) {
    var x = "bar";
  }
  print(o.x); // "bar"
}
function f() {
  var x;
  var o = {x: "foo"};
  with (o) {
    x = "bar";
  }
  print(o.x); // "bar"
} 

Note that lets still behave unsurprisingly as their declarations do not hoist outside of the with. They shadow properties of the same name.

eval may capture assignments, but not var declarations

eval'd vars hoist normally, so evals may capture assignments similar to with:

function f() {
  {
     let x = "inner";
     eval("var x = 'outer'");
     print(x); // "outer"
  }
}

for heads

  • vars in for heads hoist to the top of the function. The following two examples are equivalent.
function f() {
  for (var i = 0; i < c; i++) {
    ...
  }
}
function f() {
  var i;
  for (i = 0; i < c; i++) {
    ...
  }
}

So it is not safe to nest vars of the same name in for heads, even if it is your intention to shadow the variable from the outer loop.

  • lets in for heads create an implicit block around the condition, update, and body parts of the for loop. The following two examples are equivalent.
function f() {
  for (let i = 0; i < c; i++) {
    ...
  }
}
function f() {
  {
    let i;
    for (i = 0; i < c; i++) {
      ...
    }
  }
}

There is no new let every iteration. There is one let around the entire loop. This behavior might change in the future: https://bugzilla.mozilla.org/show_bug.cgi?id=449811

catch variables are block-scoped

Variables that are caught in catch blocks are block-scoped, like lets.

function f() {
  try {
    throw "foo";
  } catch (e) {
  }
  // e undefined here
}

let statements and expressions

  • let statements creates bindings in the accompanying block.
function f() {
  let (x) {
    x = "foo";
    print(x); // "foo"
  }
  // x is undefined here
  let (x = "bar") {
    print(x); // "bar"
  }
  // x is undefined here
}
  • let expressions creates binding in the accompanying expression.
function f() {
  (1 + (let (i = 1) i)); // 2
  ((let (i = 1) i) + i); // error, second use of i is unbound
}

function oddities

  • functions do not hoist when declared inside a child block.
function f() {
  {
    g(); // error, g undefined
    function g() {
      ...
    }
  }
}
  • "dynamic scope", where the scope of the parent function in which an inner function is defined can be mutated at run-time.
function g() {
  print("global");
}
function f(cond) {
  if (cond) {
    function g() {
      print("inner");
   }
  }
  g(); // "inner" when cond, "global" when !cond
}
  • Named function expressions are expression-scoped. Their names are only bound inside the expression in which they're defined. They also don't mutate the existing scope.
function f() {
  (function g() { print("g"); })();
  g(); // error, g undefined
}
  • Functions initializations happen at the top of the parent function (above vars). Since vars declarations with names already existent as a parameter or a function are no-ops, we get some surprising results.
function f() {
  function g() {
    print("foo");
  }
  var g;
  g(); // "foo"
}
function f() {
  var g = 0;
  function g() {
    print("foo");
  }
  g(); // error, not a function because the function g's initialization to the function is overwritten by its assignment to 0
}
  • Functions are not hoisted at all if they're inside a block, but they can still mutate existing scope.
function f() {
  var g = 0;
  if (cond) {
    function g() {
      print("foo");
    }
  }
  g(); // prints "foo" when cond, error when !cond
}

E4X selector predicates

E4X selector predicates add an XML item to the scope chain to evaluate the filter expression.

list = <><item><name>foo</name></item><item><name>bar</name></item><item><name>baz</name></item></>;
subList = list.(String(name) === "bar")

Document Tags and Contributors

Tags: 
  • hoisting
  • JavaScript
  • scope
 Contributors to this page: fscholz, rogerhc, getify, Sheppy, syg, Dherman
 Last updated by: fscholz, Jul 1, 2015, 3:05:25 PM

  1. .htaccess ( hypertext access )
  2. <input> archive
  3. Add-ons
    1. Add-ons
    2. Firefox addons developer guide
    3. Interaction between privileged and non-privileged pages
    4. Tabbed browser
    5. bookmarks.export()
    6. bookmarks.import()
  4. Adding preferences to an extension
  5. An Interview With Douglas Bowman of Wired News
  6. Apps
    1. Apps
    2. App Development API Reference
    3. Designing Open Web Apps
    4. Graphics and UX
    5. Open web app architecture
    6. Tools and frameworks
    7. Validating web apps with the App Validator
  7. Archived Mozilla and build documentation
    1. Archived Mozilla and build documentation
    2. ActiveX Control for Hosting Netscape Plug-ins in IE
    3. Archived SpiderMonkey docs
    4. Autodial for Windows NT
    5. Automated testing tips and tricks
    6. Automatic Mozilla Configurator
    7. Automatically Handle Failed Asserts in Debug Builds
    8. BlackConnect
    9. Blackwood
    10. Bonsai
    11. Bookmark Keywords
    12. Building TransforMiiX standalone
    13. Chromeless
    14. Creating a Firefox sidebar extension
    15. Creating a Microsummary
    16. Creating a Mozilla Extension
    17. Creating a Release Tag
    18. Creating a Skin for Firefox/Getting Started
    19. Creating a Skin for Mozilla
    20. Creating a Skin for SeaMonkey 2.x
    21. Creating a hybrid CD
    22. Creating regular expressions for a microsummary generator
    23. DTrace
    24. Dehydra
    25. Developing New Mozilla Features
    26. Devmo 1.0 Launch Roadmap
    27. Download Manager improvements in Firefox 3
    28. Download Manager preferences
    29. Drag and Drop
    30. Embedding FAQ
    31. Embedding Mozilla in a Java Application using JavaXPCOM
    32. Error Console
    33. Exception logging in JavaScript
    34. Existing Content
    35. Extension Frequently Asked Questions
    36. Fighting Junk Mail with Netscape 7.1
    37. Firefox Sync
    38. Force RTL
    39. GRE
    40. Gecko Coding Help Wanted
    41. HTTP Class Overview
    42. Hacking wiki
    43. Help Viewer
    44. Helper Apps (and a bit of Save As)
    45. Hidden prefs
    46. How to Write and Land Nanojit Patches
    47. Introducing the Audio API extension
    48. Java in Firefox Extensions
    49. JavaScript crypto
    50. Jetpack
    51. Litmus tests
    52. Makefile.mozextension.2
    53. Microsummary topics
    54. Migrate apps from Internet Explorer to Mozilla
    55. Monitoring downloads
    56. Mozilla Application Framework
    57. Mozilla Crypto FAQ
    58. Mozilla Modules and Module Ownership
    59. Mozprocess
    60. Mozprofile
    61. Mozrunner
    62. Nanojit
    63. New Skin Notes
    64. Persona
    65. Plug-n-Hack
    66. Plugin Architecture
    67. Porting NSPR to Unix Platforms
    68. Priority Content
    69. Prism
    70. Proxy UI
    71. Remote XUL
    72. SXSW 2007 presentations
    73. Space Manager Detailed Design
    74. Space Manager High Level Design
    75. Standalone XPCOM
    76. Stress testing
    77. Structure of an installable bundle
    78. Supporting private browsing mode
    79. Table Cellmap
    80. Table Cellmap - Border Collapse
    81. Table Layout Regression Tests
    82. Table Layout Strategy
    83. Tamarin
    84. The Download Manager schema
    85. The life of an HTML HTTP request
    86. The new nsString class implementation (1999)
    87. TraceVis
    88. Treehydra
    89. URIScheme
    90. URIs and URLs
    91. Using Monotone With Mozilla CVS
    92. Using SVK With Mozilla CVS
    93. Using addresses of stack variables with NSPR threads on win16
    94. Venkman
    95. Video presentations
    96. Why Embed Gecko
    97. XML in Mozilla
    98. XPInstall
    99. XPJS Components Proposal
    100. XRE
    101. XTech 2005 Presentations
    102. XTech 2006 Presentations
    103. XUL Explorer
    104. XULRunner
    105. ant script to assemble an extension
    106. calICalendarView
    107. calICalendarViewController
    108. calIFileType
    109. xbDesignMode.js
  8. Archived open Web documentation
    1. Archived open Web documentation
    2. Browser Detection and Cross Browser Support
    3. Browser Feature Detection
    4. Displaying notifications (deprecated)
    5. E4X
    6. E4X Tutorial
    7. LiveConnect
    8. MSX Emulator (jsMSX)
    9. Old Proxy API
    10. Properly Using CSS and JavaScript in XHTML Documents
    11. Reference
    12. Scope Cheatsheet
    13. Server-Side JavaScript
    14. Sharp variables in JavaScript
    15. Standards-Compliant Authoring Tools
    16. Using JavaScript Generators in Firefox
    17. Window.importDialog()
    18. Writing JavaScript for XHTML
    19. XForms
    20. background-size
    21. forEach
  9. B2G OS
    1. B2G OS
    2. Automated Testing of B2G OS
    3. B2G OS APIs
    4. B2G OS add-ons
    5. B2G OS architecture
    6. B2G OS build prerequisites
    7. B2G OS phone guide
    8. Building B2G OS
    9. Building and installing B2G OS
    10. Building the B2G OS Simulator
    11. Choosing how to run Gaia or B2G
    12. Customization with the .userconfig file
    13. Debugging on Firefox OS
    14. Developer Mode
    15. Developing Firefox OS
    16. Firefox OS Simulator
    17. Firefox OS apps
    18. Firefox OS board guide
    19. Firefox OS developer release notes
    20. Firefox OS security
    21. Firefox OS usage tips
    22. Gaia
    23. Installing B2G OS on a mobile device
    24. Introduction to Firefox OS
    25. Mulet
    26. Open web apps quickstart
    27. Pandaboard
    28. PasscodeHelper Internals
    29. Porting B2G OS
    30. Preparing for your first B2G build
    31. Resources
    32. Running tests on Firefox OS: A guide for developers
    33. The B2G OS platform
    34. Troubleshooting B2G OS
    35. Using the App Manager
    36. Using the B2G emulators
    37. Web Bluetooth API (Firefox OS)
    38. Web Telephony API
    39. Web applications
  10. Beginner tutorials
    1. Beginner tutorials
    2. Creating reusable content with CSS and XBL
    3. Underscores in class and ID Names
    4. XML data
    5. XUL user interfaces
  11. Case Sensitivity in class and id Names
  12. Creating a dynamic status bar extension
  13. Creating a status bar extension
  14. Gecko Compatibility Handbook
  15. Getting the page URL in NPAPI plugin
  16. Index
  17. Inner-browsing extending the browser navigation paradigm
  18. Install.js
  19. JXON
  20. List of Former Mozilla-Based Applications
  21. List of Mozilla-Based Applications
  22. Localizing an extension
  23. MDN
    1. MDN
    2. Content kits
  24. MDN "meta-documentation" archive
    1. MDN "meta-documentation" archive
    2. Article page layout guide
    3. Blog posts to integrate into documentation
    4. Current events
    5. Custom CSS classes for MDN
    6. Design Document
    7. DevEdge
    8. Developer documentation process
    9. Disambiguation
    10. Documentation Wishlist
    11. Documentation planning and tracking
    12. Editing MDN pages
    13. Examples
    14. Existing Content/DOM in Mozilla
    15. External Redirects
    16. Finding the right place to document bugs
    17. Getting started as a new MDN contributor
    18. Landing page layout guide
    19. MDN content on WebPlatform.org
    20. MDN page layout guide
    21. MDN subproject list
    22. Needs Redirect
    23. Page types
    24. RecRoom documentation plan
    25. Remove in-content iframes
    26. Team status board
    27. Trello
    28. Using the Mozilla Developer Center
    29. Welcome to the Mozilla Developer Network
    30. Writing chrome code documentation plan
    31. Writing content
  25. MMgc
  26. Makefile - .mk files
  27. Marketplace
    1. Marketplace
    2. API
    3. Monetization
    4. Options
    5. Publishing
  28. Mozilla release FAQ
  29. Newsgroup summaries
    1. Newsgroup summaries
    2. Format
    3. Mozilla.dev.apps.firefox-2006-09-29
    4. Mozilla.dev.apps.firefox-2006-10-06
    5. mozilla-dev-accessibility
    6. mozilla-dev-apps-calendar
    7. mozilla-dev-apps-firefox
    8. mozilla-dev-apps-thunderbird
    9. mozilla-dev-builds
    10. mozilla-dev-embedding
    11. mozilla-dev-extensions
    12. mozilla-dev-i18n
    13. mozilla-dev-l10n
    14. mozilla-dev-planning
    15. mozilla-dev-platform
    16. mozilla-dev-quality
    17. mozilla-dev-security
    18. mozilla-dev-tech-js-engine
    19. mozilla-dev-tech-layout
    20. mozilla-dev-tech-xpcom
    21. mozilla-dev-tech-xul
    22. mozilla.dev.apps.calendar
    23. mozilla.dev.tech.js-engine
  30. Obsolete: XPCOM-based scripting for NPAPI plugins
  31. Plugins
    1. Plugins
    2. Adobe Flash
    3. External resources for plugin creation
    4. Logging Multi-Process Plugins
    5. Monitoring plugins
    6. Multi-process plugin architecture
    7. NPAPI plugin developer guide
    8. NPAPI plugin reference
    9. Samples and Test Cases
    10. Shipping a plugin as a Toolkit bundle
    11. Supporting private browsing in plugins
    12. The First Install Problem
    13. Writing a plugin for Mac OS X
    14. XEmbed Extension for Mozilla Plugins
  32. SAX
  33. Security
    1. Security
    2. Digital Signatures
    3. Encryption and Decryption
    4. Introduction to Public-Key Cryptography
    5. Introduction to SSL
    6. NSPR Release Engineering Guide
    7. SSL and TLS
  34. Solaris 10 Build Prerequisites
  35. Sunbird Theme Tutorial
  36. Table Reflow Internals
  37. Tamarin Tracing Build Documentation
  38. The Basics of Web Services
  39. Themes
    1. Themes
    2. Building a Theme
    3. Common Firefox theme issues and solutions
    4. Creating a Skin for Firefox
    5. Making sure your theme works with RTL locales
    6. Theme changes in Firefox 2
    7. Theme changes in Firefox 3
    8. Theme changes in Firefox 3.5
    9. Theme changes in Firefox 4
  40. Updating an extension to support multiple Mozilla applications
  41. Using IO Timeout And Interrupt On NT
  42. Using SSH to connect to CVS
  43. Using workers in extensions
  44. WebVR
    1. WebVR
    2. WebVR environment setup
  45. XQuery
  46. XUL Booster
  47. XUL Parser in Python