Parser Hooks

The parser instance, found in the compiler, is used to parse each module being processed by webpack. The parser is yet another webpack class that extends tapable and provides a variety of tapable hooks that can be used by plugin authors to customize the parsing process.

The parser is found within module factories and therefore takes little more work to access:

compiler.hooks.normalModuleFactory.tap('MyPlugin', factory => {
  factory.hooks.parser.for('javascript/auto').tap('MyPlugin', (parser, options) => {
    parser.hooks.someHook.tap(/* ... */);
  });
});

As with the compiler, tapAsync and tapPromise may also be available depending on the type of hook.

Hooks

The following lifecycle hooks are exposed by the parser and can be accessed as such:

evaluateTypeof

SyncBailHook

Triggered when evaluating an expression consisting in a typeof of a free variable

• Hook Parameters: identifier
• Callback Parameters: expression

parser.hooks.evaluateTypeof.for('myIdentifier').tap('MyPlugin', expression => {
  /* ... */
  return expressionResult;
});

This will trigger the evaluateTypeof hook:

const a = typeof myIdentifier;

This won't trigger:

const myIdentifier = 0;
const b = typeof myIdentifier;

evaluate

SyncBailHook

Called when evaluating an expression.

• Hook parameters: expressionType
• Callback parameters: expression

For example:

index.js

const a = new String();

MyPlugin.js

parser.hooks.evaluate.for('NewExpression').tap('MyPlugin', expression => {
  /* ... */
  return expressionResult;
});

Where the expressions types are:

• 'ArrowFunctionExpression'
• 'AssignmentExpression'
• 'AwaitExpression'
• 'BinaryExpression'
• 'CallExpression'
• 'ClassExpression'
• 'ConditionalExpression'
• 'FunctionExpression'
• 'Identifier'
• 'LogicalExpression'
• 'MemberExpression'
• 'NewExpression'
• 'ObjectExpression'
• 'SequenceExpression'
• 'SpreadElement'
• 'TaggedTemplateExpression'
• 'TemplateLiteral'
• 'ThisExpression'
• 'UnaryExpression'
• 'UpdateExpression'

evaluateIdentifier

SyncBailHook

Called when evaluating an identifier that is a free variable.

• Hook Parameters: identifier
• Callback Parameters: expression

evaluateDefinedIdentifier

SyncBailHook

Called when evaluating an identifier that is a defined variable.

• Hook Parameters: identifier
• Callback Parameters: expression

evaluateCallExpressionMember

SyncBailHook

Called when evaluating a call to a member function of a successfully evaluated expression.

• Hook Parameters: identifier
• Callback Parameters: expression param

This expression will trigger the hook:

index.js

const a = expression.myFunc();

MyPlugin.js

parser.hooks.evaluateCallExpressionMember.for('myFunc').tap('MyPlugin', (expression, param) => {
  /* ... */
  return expressionResult;
});

statement

SyncBailHook

General purpose hook that is called for every parsed statement in a code fragment.

• Callback Parameters: statement

parser.hooks.statement.tap('MyPlugin', statement => { /* ... */ });

Where the statement.type could be:

• 'BlockStatement'
• 'VariableDeclaration'
• 'FunctionDeclaration'
• 'ReturnStatement'
• 'ClassDeclaration'
• 'ExpressionStatement'
• 'ImportDeclaration'
• 'ExportAllDeclaration'
• 'ExportDefaultDeclaration'
• 'ExportNamedDeclaration'
• 'IfStatement'
• 'SwitchStatement'
• 'ForInStatement'
• 'ForOfStatement'
• 'ForStatement'
• 'WhileStatement'
• 'DoWhileStatement'
• 'ThrowStatement'
• 'TryStatement'
• 'LabeledStatement'
• 'WithStatement'

statementIf

SyncBailHook

Called when parsing an if statement. Same as the statement hook, but triggered only when statement.type == 'IfStatement'.

• Callback Parameters: statement

label

SyncBailHook

Called when parsing statements with a label. Those statements have statement.type === 'LabeledStatement'.

• Hook Parameters: labelName
• Callback Parameters: statement

import

SyncBailHook

Called for every import statement in a code fragment. The source parameter contains the name of the imported file.

• Callback Parameters: statement source

The following import statement will trigger the hook once:

index.js

import _ from 'lodash';

MyPlugin.js

parser.hooks.import.tap('MyPlugin', (statement, source) => {
  // source == 'lodash'
});

importSpecifier

SyncBailHook

Called for every specifier of every import statement.

• Callback Parameters: statement source exportName identifierName

The following import statement will trigger the hook twice:

index.js

import _, { has } from 'lodash';

MyPlugin.js

parser.hooks.importSpecifier.tap('MyPlugin', (statement, source, exportName, identifierName) => {
  /* First call
    source == 'lodash'
    exportName == 'default'
    identifierName == '_'
  */
  /* Second call
    source == 'lodash'
    exportName == 'has'
    identifierName == 'has'
  */
});

export

SyncBailHook

Called for every export statement in a code fragment.

• Callback Parameters: statement

exportImport

SyncBailHook

Called for every export-import statement eg: export * from 'otherModule';.

• Callback Parameters: statement source

exportDeclaration

SyncBailHook

Called for every export statement exporting a declaration.

• Callback Parameters: statement declaration

Those exports will trigger this hook:

export const myVar = 'hello'; // also var, let
export function FunctionName(){}
export class ClassName {}

exportExpression

SyncBailHook

Called for every export statement exporting an expression e.g.export default expression;.

• Callback Parameters: statement declaration

exportSpecifier

SyncBailHook

Called for every specifier of every export statement.

• Callback Parameters: statement identifierName exportName index

exportImportSpecifier

SyncBailHook

Called for every specifier of every export-import statement.

• Callback Parameters: statement source identifierName exportName index

varDeclaration

SyncBailHook

Called when parsing a variable declaration.

• Callbak Parameters: declaration

varDeclarationLet

SyncBailHook

Called when parsing a variable declaration defined using let

• Callbak Parameters: declaration

varDeclarationConst

SyncBailHook

Called when parsing a variable declaration defined using const

• Callbak Parameters: declaration

varDeclarationVar

SyncBailHook

Called when parsing a variable declaration defined using var

• Callbak Parameters: declaration

canRename

SyncBailHook

Triggered before renaming an identifier to determine if the renaming is allowed. This is usually used together with the rename hook.

• Hook Parameters: identifier
• Callback Parameters: expression

var a = b;

parser.hooks.canRename.for('b').tap('MyPlugin', expression => {
  // returning true allows renaming
  return true;
});

rename

SyncBailHook

Triggered when renaming to get the new identifier. This hook will be called only if canRename returns true.

• Hook Parameters: identifier
• Callback Parameters: expression

var a = b;

parser.hooks.rename.for('b').tap('MyPlugin', expression => {});

assigned

SyncBailHook

Called when parsing an AssignmentExpression before parsing the assigned expression.

• Hook Parameters: identifier
• Callback Parameters: expression

a += b;

parser.hooks.assigned.for('a').tap('MyPlugin', expression => {
  // this is called before parsing b
});

assign

SyncBailHook

Called when parsing an AssignmentExpression before parsing the assign expression.

• Hook Parameters: identifier
• Callback Parameters: expression

a += b;

parser.hooks.assigned.for('a').tap('MyPlugin', expression => {
  // this is called before parsing a
});

typeof

SyncBailHook

Triggered when parsing the typeof of an identifier

• Hook Parameters: identifier
• Callback Parameters: expression

call

SyncBailHook

Called when parsing a function call.

• Hook Parameters: identifier
• Callback Parameters: expression

eval(/* something */);

parser.hooks.call.for('eval').tap('MyPlugin', expression => {});

callAnyMember

SyncBailHook

Triggered when parsing a call to a member function of an object.

• Hook Parameters: objectIdentifier
• Callback Parameters: expression

myObj.anyFunc();

parser.hooks.callAnyMember.for('myObj').tap('MyPlugin', expression => {});

new

SyncBailHook

Invoked when parsing a new expression.

• Hook Parameters: identifier
• Callback Parameters: expression

new MyClass();

parser.hooks.new.for('MyClass').tap('MyPlugin', expression => {});

expression

SyncBailHook

Called when parsing an expression.

• Hook Parameters: identifier
• Callback Parameters: expression

const a = this;

parser.hooks.new.for('this').tap('MyPlugin', expression => {});

expressionAnyMember

SyncBailHook

Executed when parsing a MemberExpression.

• Hook Parameters: identifier
• Callback Parameters: expression

const a = process.env;

parser.hooks.new.for('process').tap('MyPlugin', expression => {});

expressionConditionalOperator

SyncBailHook

Called when parsing a ConditionalExpression e.g. condition ? a : b

• Callback Parameters: expression

program

SyncBailHook

Get access to the abstract syntax tree (AST) of a code fragment

Parameters: ast comments