How to initialize empty constructor with nested object?

Question

I am having the following problem with my code: I cannot figure out how to initialize nestedStudents. Every time I try to do it, it just tells me that that the object might be undefined.

export class StudentDto {
  students: number; 
  nestedStudents: {
    grades: number;
    classes: number;
  }

  constructor(
    props: {
        students?: number;
        nestedStudents?: {
          grades?: number
          classes?: number
        }
    } = {},
  ) {
    this.students = props.students || 0;
    //How can I initialize nestedStudents here?
  }
}

Kelvin Schoofs · Accepted Answer · 2021-08-06 18:26:15Z

1

You can do something similar to your statement for students, but replace the 0 with something like this:

  constructor(
    props: {
        students?: number;
        nestedStudents?: {
          grades?: number
          classes?: number
        }
    } = {},
  ) {
    this.students = props.students || 0;
    this.nestedStudents =  || {
        grades: 0, // default value
        classes: 0, // default value
        ...props.nestedStudent, // overwrite with what's given
        // ^ doing `...(undefined)` is fine, it just does nothing
    };
  }

edited Aug 6, 2021 at 18:26

answered Aug 6, 2021 at 18:21

Kelvin Schoofs

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6 Comments

lakeIn231 Over a year ago

I tried this, but it is giving me the error: type '{ grades?: number | undefined; classes?: number | undefined; }' is not assignable to type '{ grades: number; classes: number; }'. Types of property 'grades' are incompatible. Type 'number | undefined' is not assignable to type 'number'. Type 'undefined' is not assignable to type 'number'.

Kelvin Schoofs Over a year ago

Oh I see, I was basing myself on your constructor parameter's type, let me update my answer.

lakeIn231 Over a year ago

Hmmm, latest update says: "Spread types may only be created from object types"

Kelvin Schoofs Over a year ago

Huh, I guess TypeScript doesn't like ...(undefined) even though JS allows it. You could use ...props.nestedStudent! to trick TS into thinking it isn't undefined, or use ...(props.nestedStudent || {}) instead.

lakeIn231 Over a year ago

Very odd. VS Code and the compiler is giving me back to what the first error is with trying either of those. Tried to overwrite with !, but still the same error

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