额外检查 Linter Checks

noFallthroughCasesInSwitch

默认：false
报告switch语句中失败案例的错误，确保switch语句中的任何非空情况都包括break或return。

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

noImplicitOverride

4.3 新增
当此选项被启用时，除非显式地使用override关键字，否则重写父类中的任何方法将引起错误。

class Album {
  setup() {}
}
class MLAlbum extends Album {
  override setup() {}
}
class SharedAlbum extends Album {
  setup() {}
// This member must have an 'override' modifier because it overrides a member in the base class 'Album'.
}

noImplicitReturns

默认：false
当启用时，TypeScript会检查函数中的所有代码路径，以确保它们返回一个值

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

noPropertyAccessFromIndexSignature

4.2 新增
默认：false
当开启时，通过点操作符访问的不明确属性会被警告（但是中括号语法不会被警告）。

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;
settings.quality;
// Property 'username' comes from an index signature, so it must be accessed with ['username'].
// This would need to be settings["username"];
settings.username;

noUncheckedIndexedAccess

4.1 新增
当开启时，未定义的签名访问会加上undefined类型

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: string | undefined

noUnusedLocals

默认：false
未使用的局部变量会抛错。

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

noUnusedParameters

默认：false
未使用的参数跑错。

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