TypeScript strict mode flags explained

Need to understand what TypeScript strict mode enables and which individual flags matter most.

TypeScript strict mode breakdown:

// tsconfig.json
{
  "compilerOptions": {
    "strict": true  // Enables ALL of the below:
  }
}

// 1. strictNullChecks (most impactful!)
// Without: string includes null/undefined implicitly
// With: must handle null explicitly
const name: string = null;  // Error!
const name: string | null = null;  // OK

function greet(name: string | null) {
  // name.toUpperCase();  // Error! name might be null
  if (name) {
    name.toUpperCase();  // OK after null check
  }
}

// 2. noImplicitAny
// Without: untyped params default to 'any'
// With: must annotate parameters
function add(a, b) { return a + b; }  // Error! a,b are implicit any
function add(a: number, b: number) { return a + b; }  // OK

// 3. strictFunctionTypes
// Enforces contravariant parameter types
type Handler = (event: MouseEvent) => void;
const handler: Handler = (event: Event) => {};  // Error!
// MouseEvent is more specific than Event

// 4. strictBindCallApply
// Type-checks .bind(), .call(), .apply()
function add(a: number, b: number) { return a + b; }
add.call(null, 1, 'two');  // Error! 'two' is not number

// 5. noImplicitThis
// Must annotate 'this' type in functions
function onClick(this: HTMLButtonElement) {
  this.textContent = 'Clicked';  // OK, 'this' is typed
}

// 6. alwaysStrict
// Emits 'use strict' in every file

// 7. useUnknownInCatchVariables (TS 4.4+)
try {
  something();
} catch (e) {
  // e is 'unknown', not 'any'
  if (e instanceof Error) {
    console.log(e.message);  // OK after narrowing
  }
}

Recommended additional flags:

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,  // arr[0] is T | undefined
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "exactOptionalPropertyTypes": true,
    "noFallthroughCasesInSwitch": true
  }
}

strict: true catches the most bugs per line of config. strictNullChecks alone prevents the majority of 'cannot read property of undefined' runtime errors.

TypeScript project configuration