Increase the readability of long numbers and uncommon number formats by adding underscore separators.

Add numeric separator

Shorten object properties when the property name is the same as the property value.

Collapse object property into shorthand

**The spread syntax (`...`) allows iterables like array expressions to be expanded in place where arguments are expected.**
It can used in place of `.apply`. For example `Math.min.apply(Math, args)` can be replaced with `Math.min(...args)`.


Replace `.apply()` calls with the spread operator `...`

Convert .apply() to spread syntax

Replace `array.indexOf()` checks with `array.includes()`.

Convert array.indexOf() into array.includes()

This refactoring replaces string comparison chains with an `array.includes()` call. It works for regular string comparisons with `|| ===` and for negated string comparisons with `&& !==`. When the string comparison is negated, the `.includes` call is prefixed with `!`.

## Example

```javascript
if (obj.property === "value1" 
    || obj.property === "value2" 
    || obj.property === "value3") {
    // ...
}
```
becomes
```javascript
if (["value1", "value2", "value3"].includes(obj.property)) {
    // ...
}
```



Replace `|| value === 'aString'` and `&& value !== 'aString'`  chains with `array.includes()`.

Convert string comparison chain to array.includes()

**Arrow functions are a concise syntax that can often replace the regular function syntax.**
For example, `function(x, y) { return x * y }` can be replaced with `(x, y) => { return x * y; }`.

However, arrow functions have different semantics and cannot replace regular functions in all cases.
In particular, the scope of arrow functions (including binding of `this`) is established at creation time.
This difference makes arrow functions unsuitable for using them as methods, constructors, and in `call`, `apply`, and `bind` calls.


Replace function expressions with arrow functions, a more concise syntax.

Convert function to arrow function

Convert property assignments with functions to method declarations.

Convert function to object method

**`for...of` loops over iterable objects, for example arrays or strings.**

It is easier to read than indexed `for` loops and can replace them in many cases.

### Refactoring Example
Before:
```javascript
for (let i = 0; i < elements.length; i++) {
  const element = elements[i];
  console.log(element);
}
```

After:
```javascript
for (const element of elements) {
  console.log(element);
}
```


Replace regular `for` loops and `anArray.forEach` loops with `for…of` loops.

Convert loop to for…of

Use the exponentiation operator `**` instead of `Math.pow()`.

Convert Math.pow to exponentiation operator

Convert a string to a basic template literal without expressions.

Convert string to template literal

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

Convert to destructuring assignment

**The optional chaining operator `.?` returns the value of an object property when the object is available and `undefined` otherwise.**
It is similar to the standard `.` chaining operator, with an added check if the object is defined (i.e., not [nullish](https://developer.mozilla.org/en-US/docs/Glossary/Nullish)).

The optional chaining operator lets you **write concise and safe chains of connected objects** when some of those objects can be `null` or `undefined`. Before the introduction of optional chaining in ES2020, the `&&` operator was often used to check if an object is available (`obj && obj.value`).

You can often simplify existing checks with the optional chaining pattern:
- `obj && obj.property` becomes `obj?.property`
- `obj != null && obj.property` becomes `obj?.property`
- `obj != null ? obj.property : undefined` becomes `obj?.property`
- `arr && arr[i]` becomes `arr?.[i]`
- `f && f()` becomes `f?.()`
- etc.


Replace various guard expressions with the optional chaining operator (`?.`).

Convert to optional chaining

**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

**`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

This refactoring replaces `void 0` and other constant `void` expressions without side-effects with `undefined` (unless `undefined` is declared as a local variable).

Replace `void 0` and other constant `void` expressions with `undefined`.

Replace void 0 with undefined

The `== null` check is a concise expression to identify nullish values (`null` and `undefined`).

This refactoring replaces the following combinations of longer strict equality checks with the shorter `null` comparison:

- `a === null || a === undefined` becomes `a == null`
- `b !== null && b !== undefined` becomes `b != null`
- `x.f(1, 2) === null || x.f(1, 2) === undefined` becomes `x.f(1, 2) == null`


Replace different nullish checks with `== null`.

Use == null comparison

**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.

Use nullish coalescence in default expression

**String.prototype.endsWith() checks if a string ends with another string.**

This refactoring replaces index-based single-character comparison with `endsWidth`,
e.g. `s[s.length - 1] === '/'` becomes `s.endsWith('/')`.


`string.endsWith()` checks if a string ends with another string.

Use string.endsWith()

**String.prototype.startsWith() checks if a string starts with another string.**

This refactoring replaces index-based single-character comparison with `startsWidth`,
e.g. `s[0] === '/'` becomes `s.startsWith('/')`.


`string.startsWith()` checks if a string starts with another string.

Use string.startsWith()

**Template literals are string templates that can include variables and expressions.**
They can replace string concatenation and are often easier to read. Example: `` `Hello, ${name}!` ``

### Mechanics

This refactoring supports converting the first line of concatenated strings into template literals or shortened strings:

- concatenated string literals will be combined, e.g. `"a" + "b"` will become `"ab"`
- concatenated expressions and variables will be embedded in templates, e.g. `"a" + x + "b"` will become `` `a${x}b` ``
- unnecesary escaping of quotes is removed when string are combined or transformed into template literals


Merge string and template literal concatenation into a single template literal or string.

Merge string concatenation

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:

20 JavaScript Modernizations