By default null and undefined are assignable to all types in TypeScript e.g.

let foo: number = 123;
foo = null; // Okay
foo = undefined; // Okay

This is modelled after how a lot of people write JavaScript. However like all things TypeScript allows you to be explicit about what can and cannot be assigned a null or undefined.

In strict null checking mode, null and undefined are different:

let foo = undefined;
foo = null; // NOT Okay

Let say we have a Member interface:

interface Member {
  name: string,
  age?: number

Not every member will provide their age, so the age is an optional property. It means the value of age will probably be undefined.

undefined is the root of all evil. It always lead to runtime error. We are always easy to write some code that will probably throw Error:

  .then(member: Member => {
    const stringifyAge = member.age.toString() // probably throw Cannot read property 'toString' of undefined

But in strict null checking mode, it will throw an error at compile time:

  .then(member: Member => {
    const stringifyAge = member.age.toString() // Object is possibly 'undefined'

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