[typescript] Typescript: Type 'string | undefined' is not assignable to type 'string'

When I make any property of an interface optional, I an error like following while assigning its member to some other variable

TS2322: Type 'string | undefined' is not assignable to type 'string'.   Type 'undefined' is not assignable to type 'string'.

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;//<<<Error here 

How do I get around this error?

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());

You can use the NonNullable Utility Type:

Example

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

Docs.

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,
}

To avoid the compilation error I used

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

And then validate the empty string.

Had the same issue.

I find out that react-scrips add "strict": true to tsconfig.json.

After I removed it everything works great.

Edit

Need to warn that changing this property means that you:

not being warned about potential run-time errors anymore.

as been pointed out by PaulG in comments! Thank you :)

Use "strict": false only if you fully understand what it affects!

You could make use of Typescript's optional chaining. Example:

const name = person?.name;

If the property name exists on the person object you would get its value but if not it would automatically return undefined.

You could make use of this resource for a better understanding.

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html

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.

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;

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  

if you want to have 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 question marks (?) from in front of the property names.

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

let name1: string | undefined = person.name;

And it'll be ok;

I know this is kinda late answer but another way besides yannick's answer https://stackoverflow.com/a/57062363/6603342 to use ! is to cast it as string thus telling TypeScript i am sure this is a string thus converting

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

to

let name1:string = person.name as string;

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 reassons we are using TypeScript to ensure that the type matches and avoid such errors at compile time.

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;