switch-exhaustiveness-check.mdx

description
Require switch-case statements to be exhaustive.

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🛑 This file is source code, not the primary documentation location! 🛑

See https://typescript-eslint.io/rules/switch-exhaustiveness-check for documentation.

When working with union types or enums in TypeScript, it's common to want to write a switch statement intended to contain a case for each possible type in the union or the enum. However, if the union type or the enum changes, it's easy to forget to modify the cases to account for any new types.

This rule reports when a switch statement over a value typed as a union of literals or as an enum is missing a case for any of those literal types and does not have a default clause.

Options

allowDefaultCaseForExhaustiveSwitch

Defaults to true. If set to false, this rule will also report when a switch statement has a case for everything in a union and also contains a default case. Thus, by setting this option to false, the rule becomes stricter.

When a switch statement over a union type is exhaustive, a final default case would be a form of dead code. Additionally, if a new value is added to the union type, a default would prevent the switch-exhaustiveness-check rule from reporting on the new case not being handled in the switch statement.

allowDefaultCaseForExhaustiveSwitch Caveats

It can sometimes be useful to include a redundant default case on an exhaustive switch statement if it's possible for values to have types not represented by the union type. For example, in applications that can have version mismatches between clients and servers, it's possible for a server running a newer software version to send a value not recognized by the client's older typings.

If your project has a small number of intentionally redundant default cases, you might want to use an inline ESLint disable comment for each of them.

If your project has many intentionally redundant default cases, you may want to disable allowDefaultCaseForExhaustiveSwitch and use the default-case core ESLint rule along with a satisfies never check.

requireDefaultForNonUnion

Defaults to false. If set to true, this rule will also report when a switch statement switches over a non-union type (like a number or string, for example) and that switch statement does not have a default case. Thus, by setting this option to true, the rule becomes stricter.

This is generally desirable so that number and string switches will be subject to the same exhaustive checks that your other switches are.

Examples of additional incorrect code for this rule with { requireDefaultForNonUnion: true }:

const value: number = Math.floor(Math.random() * 3);

switch (value) {
  case 0:
    return 0;
  case 1:
    return 1;
}

Since value is a non-union type it requires the switch case to have a default clause only with requireDefaultForNonUnion enabled.

Examples

When the switch doesn't have exhaustive cases, either filling them all out or adding a default will correct the rule's complaint.

Here are some examples of code working with a union of literals:

type Day =
  | 'Monday'
  | 'Tuesday'
  | 'Wednesday'
  | 'Thursday'
  | 'Friday'
  | 'Saturday'
  | 'Sunday';

declare const day: Day;
let result = 0;

switch (day) {
  case 'Monday':
    result = 1;
    break;
}

type Day =
  | 'Monday'
  | 'Tuesday'
  | 'Wednesday'
  | 'Thursday'
  | 'Friday'
  | 'Saturday'
  | 'Sunday';

declare const day: Day;
let result = 0;

switch (day) {
  case 'Monday':
    result = 1;
    break;
  case 'Tuesday':
    result = 2;
    break;
  case 'Wednesday':
    result = 3;
    break;
  case 'Thursday':
    result = 4;
    break;
  case 'Friday':
    result = 5;
    break;
  case 'Saturday':
    result = 6;
    break;
  case 'Sunday':
    result = 7;
    break;
}

type Day =
  | 'Monday'
  | 'Tuesday'
  | 'Wednesday'
  | 'Thursday'
  | 'Friday'
  | 'Saturday'
  | 'Sunday';

declare const day: Day;
let result = 0;

switch (day) {
  case 'Monday':
    result = 1;
    break;
  default:
    result = 42;
}

Likewise, here are some examples of code working with an enum:

enum Fruit {
  Apple,
  Banana,
  Cherry,
}

declare const fruit: Fruit;

switch (fruit) {
  case Fruit.Apple:
    console.log('an apple');
    break;
}

enum Fruit {
  Apple,
  Banana,
  Cherry,
}

declare const fruit: Fruit;

switch (fruit) {
  case Fruit.Apple:
    console.log('an apple');
    break;

  case Fruit.Banana:
    console.log('a banana');
    break;

  case Fruit.Cherry:
    console.log('a cherry');
    break;
}

enum Fruit {
  Apple,
  Banana,
  Cherry,
}

declare const fruit: Fruit;

switch (fruit) {
  case Fruit.Apple:
    console.log('an apple');
    break;

  default:
    console.log('a fruit');
    break;
}

When Not To Use It

If you don't frequently switch over union types or enums with many parts, or intentionally wish to leave out some parts, this rule may not be for you.