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

Convert destructuring to regular variable declaration

### Why is this refactoring helpful?
The `const` keyword prevents re-assigning the value of the variable. It makes it easier to reason about the code since re-assignment does not need to be considered.

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

Convert let to const

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

Convert 'new Array(…)' to '[…]'

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

Convert to destructuring assignment

To extract a variable, you need to **select the full expression** that you would like to extract, and then invoke the refactor or extract context menu.

Extract one or more occurrences of an expression into a `const` variable.

Extract variable

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

Flatten array rest/spread property

 **Inline return** converts a variable assignment to a `return` statement. 
 Such a conversion is possible when the following statement after the assignment in the control flow returns the variable.

 This refactoring simplifies the control flow and makes it easier to reason about a function.

Convert a variable assignment to a `return` statement.

Replace assignment with return

**Default parameters allow providing default values for function parameters. These default values are used if nothing or `undefined` is passed into the function.**
For example, `function f(p = 42) { ... }` sets `p` to `42` if no value is passed in.

Previous ways of setting default values, for example `p = p || 42` can be converted to default parameters.
This refactoring supports converting the following expressions and variants of them:

- or assignment, e.g. `p = p || 42` or `p ||= 42`
- nullish coalescing operator, e.g. `p = p ?? 42`
- ternary with falsy check on parameter, e.g. `p = p ? p : 42`
- ternary with `null` check, e.g. `p = p == null ? 42 : p` or `p = p != null ? p : 42`
- ternary with `undefined` check, e.g. `p = p === undefined ? 42 : p`
- ternary with `typeof` check, e.g. `p = typeof p === "undefined" ? 42 : p`


Replace default value assignment expressions with default parameter values.

Move default value into parameter

Combine an object destructuring assignment with its preceding sibling.

Merge into preceding destructuring assignment

Convert the initial assignment of a variable into its declaration initializer.

Merge variable declaration and initialization

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

Move constant to top-level scope

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

Extract destructured expression into separate variable declaration

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

Move field initialization into constructor

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

Move initialization into field declaration

Move variable declaration

When a variable is initialized with a second variable, and that second variable is only used in that initialization, you can use the variable instead of the second variable and remove the indirection.

For example, here `variable` can be pushed in the definition of `secondVariable`:
```javascript
const secondVariable = someSource();
const variable = secondVariable;
```
becomes
```javascript
const variable = someSource();
```

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

Push variable declaration into initial value

Push a parameter of an immediately-invoked function expressions (IIFEs) or an immediately-invoked arrow functions (IIAFs) into the function body.

Push parameter into IIFE/IIAF

Remove trailing array destructuring holes and empty array destructuring expressions.

Remove trailing array destructuring holes

Remove a variable that is not read or written.

Remove unused variable

Replace an expression with an existing variable.

Replace with existing variable

**`let` and `const` are block-scope variable declarations that can replace 'var' declarations in many cases.**

`const` declares blocked-scoped variables that cannot be re-assigned. `let` declares block-scoped variables that can be changed.
They are available since ES6 and are preferred over function-scope, modifiable `var` declarations, because they make it easier to reason about the code.

When there are several variables declared in a statement, this refactoring does not change the declaration to prevent formatting breakages and linter issues.


Replace `var` with block-scoped variables `let` and `const`.

Convert var to let or const

**Convert variable declarations with multiple variables into separate declarations that declare one variable each.**

For example,
```javascript
let a = 2, b;
```
is converted into
```javascript
let a = 2;
let b;
```

Convert declarations with multiple variables into separate declarations for each variable.

Split variable declaration sequence

Separate the variable initialization from its declaration.

Split variable declaration and initialization

**The nullish coalescing operator (`??`) returns its right side when its left side is nullish** (`null` or `undefined`), and its left side otherwise.
For example, `const x = a ?? b` would set `x` to `a` if `a` has a value, and to `b` if `a` is `null` or `undefined`.

The nullish coalescing operator is very useful to **provide default values when a value or an expression is nullish**.
Before its introduction in ES2020, this default value pattern was often expressed using the conditional operator.

This refactoring simplifies many types of default value expressions to nullish coalescing operator expressions:

- `a == null ? x : a` becomes `a ?? x`
- `a != null ? a : x` becomes `a ?? x`
- `a !== null && a !== undefined ? a : x` becomes `a ?? x`
- `f(1) != null ? f(1) : x` becomes `f(1) ?? x`
- `if (a != null) { b = a; } else { b = x; }` becomes `b = a ?? x;`
- etc.


Replace default value expression with nullish coalescing operator (`??`) expressions.

24 Code Assists for Variables