AST utility to collect scope info for variables

Usage no npm install needed!

<script type="module">
  import ecmaVariableScope from '';


ecma-variable-scope Build status Coverage Status

AST utility to collect scope info for variables

Scope detection is hard, especially when with exists. This utility extracts all relevant info for making decisions. This project was built as part of esformatter-phonetic, a esformatter plugin that makes obfuscated variable names more comprehensible.


  • Detect with usage
  • Does not mark labels
  • Support for let and const
  • Support for destructured variables (e.g. var {hello, world} = obj;)
  • Support for arrow expressions (e.g. (hello) =>

Getting Started

Install the module with: npm install ecma-variable-scope

// Gather an AST to analyze
var esprima = require('esprima');
var ecmaVariableScope = require('ecma-variable-scope');
var ast = esprima.parse([
  'function logger(str) {',

// Determine the scope of a variable
// `logger` variable
   { type: 'lexical',
     node: Program,
     parent: undefined,
     identifiers: { logger: [Circular] },
     children: [ [Object] ] },
   { insideWith: false,
     topLevel: true,
     type: 'lexical',
     usedInAWith: false }

// `console` variable
  scope: undefined,
   { insideWith: false,
     topLevel: true,
     type: 'undeclared',
     usedInAWith: false }


ecma-variable-scope exports ecmaVariableScope as its module.exports.


Walk an abstract syntax tree and add scope and scopeInfo properties to appropriate nodes.

  • ast Object - Abstract syntax tree to walk over/mutate


We return the same ast variable but with the addition of scope and scopeInfo nodes on Identifiers that are variables.

// `scope` and `scopeInfo` will be defined for all `hello`, `world`, and `moon` references
function hello(world) {
  var moon;
  hello(world, moon);

// `log` is an `Identifier` but will not have `scope` and `scopeInfo`


Object containing information about the outermost scope a variable can be accessed from:

  • type String - Variant of scope that a variable is in
    • This can be lexical or block. When we traverse scope.parent, we can run into with but this is not directly found from node.scope.
    • For example in function a() { var b; }, we have type: 'lexical' for b's scope.
    • We make these values available via exports.SCOPE_TYPES.LEXICAL ('lexical'), exports.SCOPE_TYPES.WITH ('with'), andexports.SCOPE_TYPES.BLOCK ('block').
  • node Object<Node> - AST node that corresponds to the top of scope
    • For example in function a() { var b; }, we have b.scope.node === a
  • parent Object<Scope>|undefined - Next scope containing the current scope. This can be any other type (e.g. lexical, block, with).
  • identifiers Object - Map from identifier name to Identifier reference of variables declared within this scope
    • For example in function a() { var b; }, we have a.scope.identifiers === {b: b's Identifier}
    • This will not contain identifiers within child scopes
  • children Scope[] - Array of child scopes contained by this scope
    • For example in function a() { }, Program's scope will contain a.scope

It is possible for an Identifier to have scopeInfo but not scope. For example, console is defined as a global outside of a script context. We cannot determine if it is defined or not and make the decision to leave it as undefined.


Object containing information about the variable itself:

  • insideWith Boolean - Indicator of whether a variable has a with between its declaration and its containing scope
    • true if there was a with,false if there was not one
    • For example in function a() { with (window) { document(); } }, we have a.insideWith === false and document.insideWith === true.
    • We provide exports.SCOPE_INFO_INSIDE_WITH.YES (true) and exports.SCOPE_INFO_INSIDE_WITH.NO (false).
  • toplevel Boolean - Indicator of whether a variable was declared in the Program scope or not
    • true if it was, false if it was not
    • For example in function a() { var b; }, we have a.scopeInfo.topLevel === true and b.scopeInfo.topLevel === false.
    • We provide exports.SCOPE_INFO_TOP_LEVEL.YES (true) and exports.SCOPE_INFO_TOP_LEVEL.NO (false).
  • type String - Reference to the type of scope given to a variable
    • This can vary from the containing scope (e.g. a let is scoped to a function; block to lexical)
    • This can be lexical (e.g. function), block (e.g. for loop), or undeclared (not declared in any containing scope)
    • For example in function a() { let b; }, we have a.scopeInfo.type === 'lexical' and b.scopeInfo.type === 'block'.
    • We provide exports.SCOPE_INFO_TYPES.LEXICAL ('lexical'), exports.SCOPE_INFO_TYPES.BLOCK ('block'), and exports.SCOPE_INFO_TYPES.UNDECLARED ('undeclared').
  • usedInAWith Boolean - Indicator of whether a variable has ever been referenced inside a with in its entire scope
    • true if it has been, false if it has not
    • For example in var hello; var obj = {}; with (obj) { hello; } }, we have hello1.usedInAWith === true and obj1.usedInAWith === false.
    • We provide exports.SCOPE_INFO_USED_IN_A_WITH.YES (true) and exports.SCOPE_INFO_USED_IN_A_WITH.NO (false).

If you would like to determine if a variable can be renamed without causing other problems, use usedInAWith (false), topLevel (false), and type (lexical/block).


There are a few extra properties that are thrown in for preparation of scope and scopeInfo. They could be replaced with a better algorithm but are there if you need them. If you are using them, please let us know via an issue.

  • _nearestScope - Present on every node and points to the closest scope of any type up its parents. This is useful for jumping through block scopes until reaching a lexical one.
  • _scopeType - Stored on initial declarations of identifiers. This is the same as scopeInfo.type but needs to be preserved before scopeInfo is generated.


In lieu of a formal styleguide, take care to maintain the existing coding style. Add unit tests for any new or changed functionality. Lint via grunt and test via npm test.


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Support via Gratipay


As of Nov 04 2014, Todd Wolfson has released this repository and its contents to the public domain.

It has been released under the UNLICENSE.