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When strictNullChecks is false, null and undefined are effectively ignored by the language. This can lead to unexpected errors at runtime.

When strictNullChecks is true, null and undefined have their own distinct types and you'll get a type error if you try to use them where a concrete value is expected.

For example with this TypeScript code, users.find has no guarantee that it will actually find a user, but you can write code as though it will:

// @strictNullChecks: false
// @target: ES2015
declare const loggedInUsername: string;

const users = [
  { name: "Oby", age: 12 },
  { name: "Heera", age: 32 },
];

const loggedInUser = users.find((u) => u.name === loggedInUsername);
console.log(loggedInUser.age);

// @strictNullChecks: false
// @target: ES2015
declare const loggedInUsername: string;

const users = [
  { name: "Oby", age: 12 },
  { name: "Heera", age: 32 },
];

const loggedInUser = users.find((u) => u.name === loggedInUsername);
console.log(loggedInUser.age);

Setting strictNullChecks to true will raise an error that you have not made a guarantee that the loggedInUser exists before trying to use it.

// @errors: 2339 2532 18048
// @target: ES2020
// @strictNullChecks
declare const loggedInUsername: string;

const users = [
  { name: "Oby", age: 12 },
  { name: "Heera", age: 32 },
];

const loggedInUser = users.find((u) => u.name === loggedInUsername);
console.log(loggedInUser.age);

// @errors: 2339 2532 18048
// @target: ES2020
// @strictNullChecks
declare const loggedInUsername: string;

const users = [
  { name: "Oby", age: 12 },
  { name: "Heera", age: 32 },
];

const loggedInUser = users.find((u) => u.name === loggedInUsername);
console.log(loggedInUser.age);

The second example failed because the array's find function looks a bit like this simplification:

// When strictNullChecks: true
type Array = {
  find(predicate: (value: any, index: number) => boolean): S | undefined;
};

// When strictNullChecks: false the undefined is removed from the type system,
// allowing you to write code which assumes it always found a result
type Array = {
  find(predicate: (value: any, index: number) => boolean): S;
};

// When strictNullChecks: true
type Array = {
  find(predicate: (value: any, index: number) => boolean): S | undefined;
};

// When strictNullChecks: false the undefined is removed from the type system,
// allowing you to write code which assumes it always found a result
type Array = {
  find(predicate: (value: any, index: number) => boolean): S;
};