The strict Flag and What It Enables

// strict enables:
// strictNullChecks, noImplicitAny, strictFunctionTypes,
// strictBindCallApply, strictPropertyInitialization,
// noImplicitThis, useUnknownInCatchVariables, alwaysStrict

// Without: let s: string = null; // OK
// With: let s: string = null; // Error

// Without: function fn(x) {} // x: any
// With: // Error: Parameter has implicit any type

type Fn = (x: string | number) => void;
const fn: Fn = (x: string) => {}; // Error with strict

class C {
  name: string; // Error — not initialized
  // name!: string; // OK — known to be set later
  constructor() { this.name = 'Alice'; } // OK
}

function add(a: number, b: number): number { return a + b; }
add.call(null, 1, '2'); // Error with strict

{ "compilerOptions": { "strictNullChecks": true, "noImplicitAny": true } }