TypeScript: Type 'string | undefined' is not assignable to type 'string'

You can now use the non-null assertion operator that is here exactly for your use case.

It tells TypeScript that even though something looks like it could be null, it can trust you that it's not:

let name1:string = person.name!; 
//                            ^ note the exclamation mark here  

Another way, besides yannick's answer, to use ! is to cast it as string thus telling TypeScript: I am sure this is a string, thus converting it.

let name1:string = person.name; // Error here compile time

to

let name1 = person.name as string; // No error here compile time, runtime though

This will make the error go away, but if by any chance this is not a string you will get a run-time error... which is one of the reasons we are using TypeScript to ensure that the type matches and avoid such errors at compile time.

To avoid the compilation error I used

let name1:string = person.name || '';

And then validate the empty string.

As of TypeScript 3.7 you can use nullish coalescing operator ??. You can think of this feature as a way to “fall back” to a default value when dealing with null or undefined

let name1:string = person.name ?? '';

The ?? operator can replace uses of || when trying to use a default value and can be used when dealing with booleans, numbers, etc. where || cannot be used.

As of TypeScript 4 you can use ??= assignment operator as a ??= b which is an alternative to a = a ?? b;

By your definition, Person.name can be null, but name1 cannot. There are two scenarios:

Person.name is never null

It tells the compiler you’re sure the name is not null by using !

let name1: string = person.name!;

Person.name can be null

Specify a default value in case name is null

let name1: string = person.name ?? "default name";

A more production-ready way to handle this is to actually ensure that name is present. Assuming this is a minimal example of a larger project that a group of people are involved with, you don't know how getPerson will change in the future.

if (!person.name) {
    throw new Error("Unexpected error: Missing name");
}

let name1: string = person.name;

Alternatively, you can type name1 as string | undefined, and handle cases of undefined further down. However, it's typically better to handle unexpected errors earlier on.

You can also let TypeScript infer the type by omitting the explicit type: let name1 = person.name This will still prevent name1 from being reassigned as a number, for example.

Here's a quick way to get what is happening:

When you did the following:

name? : string

You were saying to TypeScript it was optional. Nevertheless, when you did:

let name1 : string = person.name; //<<<Error here 

You did not leave it a choice. You needed to have a Union on it reflecting the undefined type:

let name1 : string | undefined = person.name; //<<<No error here 

Using your answer, I was able to sketch out the following which is basically, an Interface, a Class and an Object. I find this approach simpler, never mind if you don't.

// Interface
interface iPerson {
    fname? : string,
    age? : number,
    gender? : string,
    occupation? : string,
    get_person?: any
}

// Class Object
class Person implements iPerson {
    fname? : string;
    age? : number;
    gender? : string;
    occupation? : string;
    get_person?: any = function () {
        return this.fname;
    }
}

// Object literal
const person1 : Person = {
    fname : 'Steve',
    age : 8,
    gender : 'Male',
    occupation : 'IT'  
}

const p_name: string | undefined = person1.fname;

// Object instance 
const person2: Person = new Person();
person2.fname = 'Steve';
person2.age = 8;
person2.gender = 'Male';
person2.occupation = 'IT';

// Accessing the object literal (person1) and instance (person2)
console.log('person1 : ', p_name);
console.log('person2 : ', person2.get_person());

Try to find out what the actual value is beforehand. If person has a valid name, assign it to name1, else assign undefined.

let name1: string = (person.name) ? person.name : undefined;

Solution 1: Remove the explicit type definition

Since getPerson already returns a Person with a name, we can use the inferred type.

function getPerson(){
  let person = {name:"John"};
  return person;
}

let person = getPerson();

If we were to define person: Person we would lose a piece of information. We know getPerson returns an object with a non-optional property called name, but describing it as Person would bring the optionality back.

Solution 2: Use a more precise definition

type Require<T, K extends keyof T> = T & {
  [P in K]-?: T[P]
};

function getPerson() {
  let person = {name:"John"};
  return person;
}

let person: Require<Person, 'name'> = getPerson();
let name1:string = person.name;

Solution 3: Redesign your interface

A shape in which all properties are optional is called a weak type and usually is an indicator of bad design. If we were to make name a required property, your problem goes away.

interface Person {
  name:string,
  age?:string,
  gender?:string,
  occupation?:string,
}

You can do it like this!

let name1:string = `${person.name}`;

But remember name1 can be an empty string.

You trying to set the variable name1, which is a type set as a strict string (it must be a string) with a value from the object field name, whose value is type set as an optional string (it can be a string or undefined, because of the question sign). If you really need this behavior, you have to change the type of name1 like this:

let name1: string | undefined = person.name;

And it'll be OK;

If you want to have a nullable property, change your interface to this:

interface Person {
  name?:string | null,
  age?:string | null,
  gender?:string | null,
  occupation?:string | null,
}

If being undefined is not the case, you can remove the question marks (?) from it in front of the property names.

You can use the NonNullable Utility Type:

Example

type T0 = NonNullable<string | number | undefined>;  // string | number
type T1 = NonNullable<string[] | null | undefined>;  // string[]

Docs.

This was the only solution I found to check if an attribute is undefined that does not generate warnings:

type NotUndefined<T, K extends keyof T> = T & Record<K, Exclude<T[K], undefined>>;
function checkIfKeyIsDefined<T, K extends keyof T>(item: T, key: K): item is NotUndefined<T, K> {
    return typeof item === 'object' && item !== null && typeof item[key] !== 'undefined';
}

Usage:

interface Listing {
    order?: string
    ...
}

obj = {..., order: 'pizza'} as Listing
if(checkIfKeyIsDefined(item: obj, 'order')) {
    order.toUpperCase() // No undefined warning O.O
}

Original answer.

If you remove the <Person> casting from your getPerson function, then TypeScript will be smart enough to detect that you return an object which definitely has a name property.

So just turn:

interface Person {
  name?: string,
  age?: string,
  gender?: string,
  occupation?: string,
}

function getPerson() {
  let person = <Person>{name: 'John'};
  return person;
}

let person: Person = getPerson();
let name1: string = person.name;

Into:

interface Person {
  name?: string,
  age?: string,
  gender?: string,
  occupation?: string,
}

function getPerson() {
  let person = {name: 'John'};
  return person;
}

let person = getPerson();
let name1: string = person.name;

If you cannot do that, then you will have to use the "definite assignment assertion operator" as @yannick1976 suggested:

let name1: string = person.name!;

Adding condition using ternary operator. The purpose of this line is to assign name1 the value of person.name if it is defined, and an empty string if not.

interface Person {
  name?: string,
  age?: string,
  gender?: string,
  occupation?: string,
}

function getPerson() {
  let person = <Person>{ name: "John" };
  return person;
}

let person: Person = getPerson();
let name1: string = person.name ? person.name : ''; // Added condition

I think to use Require, as mentioned by Karol Majewski, is quite nice. Another way to achieve the same would be to use intersection types (which is actually used internally by Require):

function getPerson(): Person & {name: string} {
  const person = {name:"John"};
  return person;
}

const person = getPerson();
const name1: string = person.name;

The advantage of using Require or intersection types is that we don't overrule the TypeScript compiler as it happens for the non-null assertion operator.

You could tighten up your types. Your getPerson function says it returns a Person, which implies that every property in the resulting value may be optional/undefined. But given the context of getPerson, we can make a stronger statement: the resulting value certainly has a name property! I would refactor getPerson to be:

function getPerson() {
  return { name: 'John' } as <Person & { name: string }>;
}

Now getPerson().name will be typed as string, not undefined | string.

You could go even further, and remove the typing altogether:

function getPerson() { return { name: 'John' }; }

TypeScript will infer the type from the returned value, and again, getPerson().name registers as type string.

One thing that was happening to me in my particular situation (but it is not related with the specific problem asked here) was that I was importing the wrong type in my file, given that the type was called exactly the same, but defining different properties.

Once I imported the correct type, all the issues disappeared.

Please try the below example. This worked for me.

interface Listing {
  name: string | undefined ,
}

export class Listing {
  public listings: Listing[] = []

  constructor(){}
  ngOnInit(){
    this.listings = fakeListing;
  }
}

Update the HTML file as the below example with &&:

  <p>First Name: {{listing.name && listing.name.split(" ")[0] }}</p>