Your helper for writing modern, clean and concise code

The P42 JavaScript Assistant adds 84 automated refactorings and actions for JavaScript, TypeScript, React, and Vue to Visual Studio Code.

You can find the actions in the quick fix and refactoring context menus. They depend on the cursor position, the selected text (if any), the source code, the language type, and any available type information. Underlining with three dots suggests beneficial refactorings that you can perform. The P42 sidebar shows you suggested refactorings for your whole file.

Documentation

• Editor Integration
• Suggestion Sidebar
• Mass Refactoring
• Safety Analysis
• Configuration
• FAQ

Code Assists by Category

Code assists that belong to several categories appear more than once.

Core Refactorings

Visual Study Code already contains basic refactorings such as Rename and Extract Function. The P42 JavaScript Assistant adds additional refactorings or extended functionality such as safety checking:

• Extract selected text into variable: Extract the selected text (including expressions from template literals) into a const variable.
• Extract variable: Extract one or more occurrences of an expression into a const variable.
• Inline return: Convert a variable assignment to a return statement.
• Inline variable: Inline a variable value into its references.

Code Assists for React

In React, components often contain JSX, a syntax extension for JavaScript. P42 provides code assists that make working with JSX and React easier:

• Add {...} to JSX attribute: Add {...} to JSX attribute string literal value.
• Collapse JSX tag: Convert an empty JSX tag into a self-closing tag.
• Expand JSX tag: Expand a self-closing JSX tag.
• Extract React function component: Extract JSX element or fragment into a React Function Component.
• Remove {...} from JSX attribute: Remove {...} from a JSX attribute expression value that contains a string literal.
• Remove unnecessary JSX fragment: Replace JSX Fragments <></> that only contain a single child with that child.
• Surround with <>...</>: Wrap JSX elements in a JSX fragment <>...</>.

Code Assists for Logical Expressions

Boolean logic can be challenging to read, especially as expressions get more complex. P42 provides several refactorings that can help you simplify and tease apart logical expressions to make them easier to understand:

• Convert string comparison chain to array.includes: Replace || value === 'aString' and && value !== 'aString' chains with array.includes().
• Convert to optional chaining: Replace various guard expressions with the optional chaining operator (?.).
• Extract variable: Extract one or more occurrences of an expression into a const variable.
• Flip operator: Swap the left and right operands and update the operator if necessary.
• Invert condition: Negate the condition of an if-statement or conditional expression and swap its content.
• Pull up negation: Move the not-operator (!) out of a binary expression.
• Push down negation: Push the not-operator (!) into an expression and negate it.
• Remove double negation: Remove double negation (!!) expressions.
• Simplify binary expression: Replace binary expression with a more straightforward equivalent expression.
• Use == null comparison: Replace different nullish checks with == null.

Code Assists for Branching Statements

Branching statements such as if-else and switch are central elements in many programs. Restructuring them can increase the readability of your programs, often in combination with refactoring their conditions:

• Convert && to if statement: Convert condition && aFunction(); and similar expression statements into if statements.
• Convert conditional expression to if-else: Convert a conditional expression to an if-else statement.
• Convert if-else into conditional expression: Convert an if-else return or assignment expression into a conditional expression.
• Convert if-else to switch: Convert if-else statement chain with equality comparisons to switch statement.
• Merge nested if inside else into else-if: Nested single if statements inside else blocks can be combined into else if statements.
• Merge nested if-statements: Combine two nested if statements without additional operations into a single if-statement, using && to combine the conditions.
• Move duplicated first statement out of if-else: Move a first statement that appears in both the if and the else block out of the if-else statement.
• Move duplicated last statement out of if-else: Move a last statement that appears in both the if and the else block out of the if-else statement.
• Remove empty else block: Remove an empty 'else' block from an 'if' statement.
• Remove empty if block: Remove an empty 'if' block from an 'if' statement. Replaces it with the 'else' block when available.
• Remove redundant else if: Remove redundant else-if conditions and unreachable else statements.
• Remove unnecessary conditional expression: Replace a conditional expression with its condition or its result.
• Remove unnecessary else: Lift the else content of an if-else with a return statement to the outer indentation level.
• Separate repeated condition into nested if-else: Separate a repeated sub-condition that is fully covered into a nested if-else.
• Split if statement: Split the condition of an if statement on || or && when possible.

Code Assists for Arrays and Loops

• Convert array.indexOf() into array.includes(): Replace array.indexOf() checks with array.includes().
• Convert string comparison chain to array.includes: Replace || value === 'aString' and && value !== 'aString' chains with array.includes().
• Convert loop to .forEach: Replace regular for loops with .forEach() loops.
• Convert loop to for..of: Replace regular for loops and anArray.forEach loops with for...of loops.
• Convert loop to for: Replace for..of with a regular for loop that has an index variable.

Code Assists for Functions and Methods

• Add {…} to arrow function: Convert arrow function expression body into a block body.
• Convert function to arrow function: Replace function expressions with arrow functions, a more concise syntax.
• Convert function to object method: Convert property assignments with functions to method declarations.
• Convert named function to function expression: Converts a named function to a const declaration with a function expression.
• Lift default into parameter: Replace default value assignment expressions with default parameter values.
• Push parameter into IIFE/IIAF: Push a parameter of an immediately-invoked function expressions (IIFEs) or an immediately-invoked arrow functions (IIAFs) into the function body.
• Remove {…} from arrow function: Convert an arrow function block body into an expression body.
• Remove IIFE/IIAF: Remove immediately-invoked function expressions (IIFEs) and immediately-invoked arrow functions (IIAFs) without parameters.

Code Assists for Strings and Template Literals

• Convert string comparison chain to array.includes: Replace || value === 'aString' and && value !== 'aString' chains with array.includes().
• Convert string to template literal: Convert a string to a basic template literal without expressions.
• Convert template literal to string: Convert a simple template literal without expressions into a string.
• Extract selected text into variable: Extract the selected text (including expressions from template literals) into a const variable.
• Inline into template: Inline a string or a basic template literal into an outer template literal.
• Remove unnecessary template literal: Simplify a template literal with a single inner expression and no prefix or suffix.
• Use string.endsWith(): string.endsWith() checks if a string ends with another string.
• Use string.startsWith(): string.startsWith() checks if a string starts with another string.
• Merge string concatenation: Merge string and template literal concatenation into a single template literal or string.

Code Assists for Variables

• Convert let to const: Replace let declarations that have no re-assignment with const declarations.
• Convert to destructuring assignment: Convert a variable declaration that accesses an object property to a destructuring assignment.
• Extract variable: Extract one or more occurrences of an expression into a const variable.
• Inline return: Convert a variable assignment to a return statement.
• Lift default into parameter: Replace default value assignment expressions with default parameter values.
• Merge into preceding destructuring assignment: Combine an object destructuring assignment with its preceding sibling.
• Merge variable declaration and initialization: Convert the initial assignment of a variable into its declaration initializer.
• Move field initialization into constructor (P42 Pro): Moves assignment of the initial value of a field into the constructor of the class.
• Push parameter into IIFE/IIAF: Push a parameter of an immediately-invoked function expressions (IIFEs) or an immediately-invoked arrow functions (IIAFs) into the function body.
• Remove unused variable: Remove a variable that is not read or written.
• Convert var to let or const: Replace var with block-scoped variables let and const.
• Split variable declaration sequence: Convert declarations with multiple variables into separate declarations for each variable.
• Split variable declaration and initialization: Separate the variable initialization from its declaration.
• Use nullish coalescence in default expression: Replace default value expression with nullish coalescing operator (??) expressions.

Code Assists for Syntax Conversion

It is often annoying to make small syntactical changes by editing text. Often more than one position needs to be edited, and the code is broken during the edit, leading to incorrect errors and auto-completions that get in the way. You can execute the following syntax conversions with code assists:

• Add {…} to arrow function: Convert arrow function expression body into a block body.
• Add {...} to JSX attribute: Add {...} to JSX attribute string literal value.
• Add numeric separator: Increase the readability of long numbers and uncommon number formats by adding underscore separators.
• Collapse JSX tag: Convert an empty JSX tag into a self-closing tag.
• Collapse object property into shorthand: Shorten object properties when the property name is the same as the property value.
• Convert property access to dot notation: Convert bracket notation property access o['a'] into dot notation property access o.a.
• Convert property access to bracket notation: Convert dot notation property access o.a into bracket notation property access o['a'].
• Expand JSX tag: Expand a self-closing JSX tag.
• Expand shorthand property: Expand a shorthand object property (e.g. { a }) to a regular property (e.g. { a: a }).
• Merge variable declaration and initialization: Convert the initial assignment of a variable into its declaration initializer.
• Pull operator out of assignment: Move an operator out of an assignment into a binary expression.
• Push operator into assignment: Move an operator from a binary expression into an assignment operator, e.g., +=.
• Remove {…} from arrow function: Convert an arrow function block body into an expression body.
• Remove {...} from JSX attribute: Remove {...} from a JSX attribute expression value that contains a string literal.

ECMAScript Modernizations

The Javascript ecosystem is progressing rapidly. However, it is hard to keep codebases up-to-date with the newer JavaScript features, and codemods are not always an option due to their significant churn and potential for breakages. P42 supports both codemod-like mass code refactoring and more opportunistic code modernization for the following upgrades:

• Add numeric separator: Increase the readability of long numbers and uncommon number formats by adding underscore separators.
• Collapse object property into shorthand: Shorten object properties when the property name is the same as the property value.
• Convert .apply() to spread syntax: Replace .apply() calls with the spread operator ...
• Convert array.indexOf() into array.includes(): Replace array.indexOf() checks with array.includes().
• Convert string comparison chain to array.includes: Replace || value === 'aString' and && value !== 'aString' chains with array.includes().
• Convert function to arrow function: Replace function expressions with arrow functions, a more concise syntax.
• Convert function to object method: Convert property assignments with functions to method declarations.
• Convert loop to for..of: Replace regular for loops and anArray.forEach loops with for...of loops.
• Convert Math.pow to exponentiation operator: Use the exponentiation operator ** instead of Math.pow().
• Convert string to template literal: Convert a string to a basic template literal without expressions.
• Convert to destructuring assignment: Convert a variable declaration that accesses an object property to a destructuring assignment.
• Convert to optional chaining: Replace various guard expressions with the optional chaining operator (?.).
• Lift default into parameter: Replace default value assignment expressions with default parameter values.
• Convert var to let or const: Replace var with block-scoped variables let and const.
• Replace void 0 with undefined: Replace void 0 and other constant void expressions with undefined.
• Use == null comparison: Replace different nullish checks with == null.
• Use nullish coalescence in default expression: Replace default value expression with nullish coalescing operator (??) expressions.
• Use string.endsWith(): string.endsWith() checks if a string ends with another string.
• Use string.startsWith(): string.startsWith() checks if a string starts with another string.
• Merge string concatenation: Merge string and template literal concatenation into a single template literal or string.

Lodash Modernizations

• Replace _.every with array.every (P42 Pro): Replace Lodash _.every with array.every.
• Replace _.filter with array.filter (P42 Pro): Replace Lodash _.filter with array.filter.
• Replace _.each and _.forEach with array.forEach (P42 Pro): Replace Lodash _.each and _.forEach with array.forEach.
• Replace _.map with array.map (P42 Pro): Replace Lodash _.map with array.map.
• Replace _.noop with arrow Function (P42 Pro): Replace _.noop with () => undefined.
• Replace _.some with array.some (P42 Pro): Replace Lodash _.some with array.some.

Code Cleanups

Code cleanups remove unnecessary code. Such code can result from code churn, e.g., by applying other refactorings, adding new features, or fixing bugs. P42 shows hints and automates the cleanup for the following situations:

• Remove console.log: Remove console.log statement.
• Remove double negation: Remove double negation (!!) expressions.
• Remove empty else block: Remove an empty 'else' block from an 'if' statement.
• Remove empty if block: Remove an empty 'if' block from an 'if' statement. Replaces it with the 'else' block when available.
• Remove IIFE/IIAF: Remove immediately-invoked function expressions (IIFEs) and immediately-invoked arrow functions (IIAFs) without parameters.
• Remove redundant else if: Remove redundant else-if conditions and unreachable else statements.
• Remove unnecessary conditional expression: Replace a conditional expression with its condition or its result.
• Remove unnecessary else: Lift the else content of an if-else with a return statement to the outer indentation level.
• Remove unnecessary expression statement: Remove an expression statement that has no side-effects.
• Remove unnecessary JSX fragment: Replace JSX Fragments <></> that only contain a single child with that child.
• Remove unnecessary template literal: Simplify a template literal with a single inner expression and no prefix or suffix.
• Remove unused variable: Remove a variable that is not read or written.
• Replace void 0 with undefined: Replace void 0 and other constant void expressions with undefined.
• Simplify binary expression: Replace binary expression with a more straightforward equivalent expression.

Other Code Assists

• Insert console.log for variable: Insert a 'console.log' statement for a selected variable when possible.
• Surround with try…catch: Surround a sequence of statements in a try…catch block.

Report Bugs and Suggest Features

Please report any bugs or feature suggestions in the P42 JavaScript Assistant issue tracker.

License & Used Open Source Libraries

See DISCLAIMER.txt.