# `prefer-optional-chain`

Prefer using concise optional chain expressions instead of chained logical ands.

TypeScript 3.7 added support for the optional chain operator.
This operator allows you to safely access properties and methods on objects when they are potentially `null` or `undefined`.

```ts
type T = {
  a?: {
    b?: {
      c: string;
      method?: () => void;
    };
  };
};

function myFunc(foo: T | null) {
  return foo?.a?.b?.c;
}
// is roughly equivalent to
function myFunc(foo: T | null) {
  return foo && foo.a && foo.a.b && foo.a.b.c;
}
// or
function myFunc(foo: T | null) {
  return (((foo || {}).a || {}).b || {}).c;
}

function myFunc(foo: T | null) {
  return foo?.['a']?.b?.c;
}
// is roughly equivalent to
function myFunc(foo: T | null) {
  return foo && foo['a'] && foo['a'].b && foo['a'].b.c;
}

function myFunc(foo: T | null) {
  return foo?.a?.b?.method?.();
}
// is roughly equivalent to
function myFunc(foo: T | null) {
  return foo && foo.a && foo.a.b && foo.a.b.method && foo.a.b.method();
}
```

Because the optional chain operator _only_ chains when the property value is `null` or `undefined`, it is much safer than relying upon logical AND operator chaining `&&`; which chains on any _truthy_ value.

## Rule Details

This rule aims enforce the usage of the safer operator.

Examples of code for this rule:

<!--tabs-->

### ❌ Incorrect

```ts
foo && foo.a && foo.a.b && foo.a.b.c;
foo && foo['a'] && foo['a'].b && foo['a'].b.c;
foo && foo.a && foo.a.b && foo.a.b.method && foo.a.b.method();

(((foo || {}).a || {}).b {}).c;
(((foo || {})['a'] || {}).b {}).c;

// this rule also supports converting chained strict nullish checks:
foo &&
  foo.a != null &&
  foo.a.b !== null &&
  foo.a.b.c != undefined &&
  foo.a.b.c.d !== undefined &&
  foo.a.b.c.d.e;
```

### ✅ Correct

```ts
foo?.a?.b?.c;
foo?.['a']?.b?.c;
foo?.a?.b?.method?.();

foo?.a?.b?.c?.d?.e;
```

**Note:** there are a few edge cases where this rule will false positive. Use your best judgement when evaluating reported errors.

## When Not To Use It

If you are not using TypeScript 3.7 (or greater), then you will not be able to use this rule, as the operator is not supported.

## Further Reading

- [TypeScript 3.7 Release Notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html)
- [Optional Chaining Proposal](https://github.com/tc39/proposal-optional-chaining/)

## Attributes

- [ ] ✅ Recommended
- [ ] 🔧 Fixable
- [ ] 💭 Requires type information
