109 Code Assists and Refactorings for JavaScript and TypeScript

The P42 JavaScript Assistant adds 109 automated code assists (refactorings, quick fixes, and cleanups) to Visual Studio Code. P42 for GitHub also suggests improvements for the JavaScript code in GitHub repositories and pull requests.

This page lists the code assists by category:

• Core Refactorings
• Code Assists for Logical Expressions
• Code Assists for Branching Statements
• Code Assists for Arrays and Loops
• Code Assists for Functions and Methods
• Code Assists for Classes
• Code Assists for Strings and Template Literals
• Code Assists for Variables
• Code Assists for Object and Array Destructuring
• Code Assists for Syntax Conversion
• JavaScript Modernizations
• Code Assists for React
• Lodash Modernizations
• Code Assists for Moving Semantic Blocks
• Code Cleanups
• Other Actions

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 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 Logical Expressions

Boolean logic can be challenging to read, especially as expressions get more complex. The JavaScript Assistant 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 (?.).

• 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.

• Introduce early return / continuepro

  Change an if-statement into an early return or continue 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 if-else-if branchespro

  Move an if-else branch up or down.

• 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.

• Move switch case clausepro

  Move a case clause in a switch statement up or down.

• 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

JavaScript has several ways of defining loops and many array methods that work on the whole array. The JavaScript Assistant provides several code actions for converting between different types of for loops and for converting to more idiomatic array methods such as array.includes().

• Convert array.filter()[0] to array.find()pro

  Replace anArray.filter(…)[0] with anArray.find(…).

• 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.

• Convert loop to .map()pro

  Convert a loop with .push() into a .map() call.

• Move array element

  Move an array element up or down.

• Move object property

  Move an object property up or down.

Code Assists for Functions and Methods

Functions and methods are essential building blocks of any non-trivial program. The following code actions make it easier to work with functions, methods, and their parameters:

• 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.

• Move default value 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 Classes

• Move field initialization into constructorpro

  Moves the assignment of the initial field value into the class constructor.

• Move initialization into field declarationpro

  Moves the assignment of the initial field value into the field declaration.

Code Assists for Strings and Template Literals

Text manipulation has become more powerful with the introduction of template literals in JavaScript. The JavaScript Assistant offers several code actions to help you work with text, be it strings or 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 destructuring to regular variable declarationpro

  Convert all variables that are declared via destructuring into separate regular variable declarations.

• Convert let to const

  Replace let declarations that have no re-assignment with const declarations.

• Convert 'new Array(…)' to '[…]'pro

  Replace new Array(…) calls with […].

• 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.

• Flatten array rest/spread propertypro

  Merge a ...[] expression into the outer array literal or destructuring expression.

• Inline return

  Convert a variable assignment to a return statement.

• Move default value 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 const to top-level scopepro

  Move a constant to the top-level scope of the module.

• Move destructured expression into separate statementpro

  Move a destructured expression inside a variable declaration into a separate variable declaration.

• Move field initialization into constructorpro

  Moves the assignment of the initial field value into the class constructor.

• Move initialization into field declarationpro

  Moves the assignment of the initial field value into the field declaration.

• Move variable declarationpro

  Move a variable declaration up or down.

• Push variable declaration into initial valuepro

  Inlines a variable that is initialized with another variable into the declaration of that variable.

• 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 trailing array destructuring holes

  Remove trailing array destructuring holes and empty array destructuring expressions.

• Remove unused variable

  Remove a variable that is not read or written.

• Replace with existing variablepro

  Replace an expression with an existing variable.

• 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 Object and Array Destructuring

• Convert destructuring to regular variable declarationpro

  Convert all variables that are declared via destructuring into separate regular variable declarations.

• Convert to destructuring assignment

  Convert a variable declaration that accesses an object property to a destructuring assignment.

• Merge into preceding destructuring assignment

  Combine an object destructuring assignment with its preceding sibling.

• Move destructuring array element

  Move an element inside an array destructuring expression up or down.

• Move destructured expression into separate statementpro

  Move a destructured expression inside a variable declaration into a separate variable declaration.

• Move destructuring object property

  Move a property inside an object destructuring expression up or down.

• Remove trailing array destructuring holes

  Remove trailing array destructuring holes and empty array destructuring 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.

JavaScript 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. The JavaScript Assistant 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 (?.).

• Move default value 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.

Code Assists for React

In React, components often contain JSX, a syntax extension for JavaScript. The JavaScript Assistant 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.

• Move JSX attributepro

  Move a JSX attribute up or down.

• 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 <>...</>.

Lodash Modernizations

With the introduction of various collection helpers and new syntax in ES6 and more recent JavaScript versions, some Lodash functions have become somewhat redundant.

• Replace _.every with array.everypro

  Replace Lodash _.every with array.every.

• Replace _.filter with array.filterpro

  Replace Lodash _.filter with array.filter.

• Replace _.each and _.forEach with array.forEachpro

  Replace Lodash _.each and _.forEach with array.forEach.

• Replace _.map with array.mappro

  Replace Lodash _.map with array.map.

• Replace _.noop with arrow Functionpro

  Replace _.noop with () => undefined.

• Replace _.some with array.somepro

  Replace Lodash _.some with array.some.

Code Assists for Moving Semantic Blocks

• Move array element

  Move an array element up or down.

• Move class memberpro

  Move a property, constructor, method, etc. up or down.

• Move const to top-level scopepro

  Move a constant to the top-level scope of the module.

• Move destructuring array element

  Move an element inside an array destructuring expression up or down.

• Move destructured expression into separate statementpro

  Move a destructured expression inside a variable declaration into a separate variable declaration.

• Move destructuring object property

  Move a property inside an object destructuring expression up or down.

• Move field initialization into constructorpro

  Moves the assignment of the initial field value into the class constructor.

• Move initialization into field declarationpro

  Moves the assignment of the initial field value into the field declaration.

• 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 if-else-if branchespro

  Move an if-else branch up or down.

• Move JSX attributepro

  Move a JSX attribute up or down.

• 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.

• Move object property

  Move an object property up or down.

• Move statement

  Move a statement up or down.

• Move switch case clausepro

  Move a case clause in a switch statement up or down.

• Move variable declarationpro

  Move a variable declaration up or down.

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. The JavaScript Assistant shows hints and automates the cleanup for the following situations:

• Flatten array rest/spread propertypro

  Merge a ...[] expression into the outer array literal or destructuring expression.

• 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 trailing array destructuring holes

  Remove trailing array destructuring holes and empty array destructuring expressions.

• 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 Actions

• Insert console.log for variable

  Insert a 'console.log' statement for a selected variable when possible.

• Insert else statement

  Add an else statement to an existing if statement

• Select expression occurrencespro

  Start a multi-cursor selection on several occurrences of the same expression.

• Surround with if statement

  Surround a sequence of statements with an if-statement.

• Surround with try…catch

  Surround a sequence of statements with a try…catch block.