Linter Checks

A collection of extra checks, which somewhat cross the boundaries of compiler vs linter. You may prefer to use a tool like eslint over these options if you are looking for more in-depth rules.

No Fallthrough Cases In Switch - noFallthroughCasesInSwitch

Report errors for fallthrough cases in switch statements. Ensures that any non-empty case inside a switch statement includes either break or return. This means you won’t accidentally ship a case fallthrough bug.

const a: number = 6;
switch (a) {
  case 0:
Fallthrough case in switch.7029Fallthrough case in switch.
    console.log("even");
  case 1:
    console.log("odd");
    break;
}Try

• Default:

  false

• Released:

  1.8

No Implicit Returns - noImplicitReturns

When enabled, TypeScript will check all code paths in a function to ensure they return a value.

function lookupHeadphonesManufacturer(color: "blue" | "black"): string {
Function lacks ending return statement and return type does not include 'undefined'.2366Function lacks ending return statement and return type does not include 'undefined'.
  if (color === "blue") {
    return "beats";
  } else {
    "bose";
  }
}Try

• Default:

  false

• Released:

  1.8

noPropertyAccessFromIndexSignature - noPropertyAccessFromIndexSignature

This setting ensures consistency between accessing a field via the “dot” (obj.key) syntax, and “indexed” (obj["key"]) and the way which the property is declared in the type.

Without this flag, TypeScript will allow you to use the dot syntax to access fields which are not defined:

interface GameSettings {
  // Known up-front properties
  speed: "fast" | "medium" | "slow";
  quality: "high" | "low";
  // Assume anything unknown to the interface
  // is a string.
  [key: string]: string;
}
const settings = getSettings();
settings.speed;
//       ^ = (property) GameSettings.speed: "fast" | "medium" | "slow"
settings.quality;
//       ^ = (property) GameSettings.quality: "high" | "low"
// Unknown key accessors are allowed on
// this object, and are `string`
settings.username;
//       ^ = stringTry

Turning the flag on will raise an error because the unknown field uses dot syntax instead of indexed syntax.

const settings = getSettings();
settings.speed;
settings.quality;
// This would need to be settings["username"];
settings.username;
Property 'username' comes from an index signature, so it must be accessed with ['username'].4111Property 'username' comes from an index signature, so it must be accessed with ['username'].//       ^ = stringTry

The goal of this flag is to signal intent in your calling syntax about how certain you are this property exists.

• Default:

  false

• Released:

  4.2

noUncheckedIndexedAccess - noUncheckedIndexedAccess

TypeScript has a way to describe objects which have unknown keys but known values on an object, via index signatures.

interface EnvironmentVars {
  NAME: string;
  OS: string;
  // Unknown properties are covered by this index signature.
  [propName: string]: string;
}
declare const env: EnvironmentVars;
// Declared as existing
const sysName = env.NAME;
const os = env.OS;
//    ^ = const os: string
// Not declared, but because of the index
// signature, then it is considered a string
const nodeEnv = env.NODE_ENV;
//    ^ = const nodeEnv: stringTry

Turning on noUncheckedIndexedAccess will add undefined to any un-declared field in the type.

declare const env: EnvironmentVars;
// Declared as existing
const sysName = env.NAME;
const os = env.OS;
//    ^ = const os: string
// Not declared, but because of the index
// signature, then it is considered a string
const nodeEnv = env.NODE_ENV;
//    ^ = const nodeEnv: string | undefinedTry

• Released:

  4.1

No Unused Locals - noUnusedLocals

Report errors on unused local variables.

const createKeyboard = (modelID: number) => {
  const defaultModelID = 23;
'defaultModelID' is declared but its value is never read.6133'defaultModelID' is declared but its value is never read.
  return { type: "keyboard", modelID };
};Try

• Default:

  false

• Released:

  2.0

No Unused Parameters - noUnusedParameters

Report errors on unused parameters in functions.

const createDefaultKeyboard = (modelID: number) => {
'modelID' is declared but its value is never read.6133'modelID' is declared but its value is never read.
  const defaultModelID = 23;
  return { type: "keyboard", modelID: defaultModelID };
};Try

• Default:

  false

• Released:

  2.0