TypeScript: does not match type error

Question

TypeScript: does not match type error

I am new to using TypeScript with Angular 2. I am currently utilizing version 1 of Angular 2 CLI. During compilation, I encountered an error stating that "is not assignable to type Assignment[]". I have reviewed the data types and everything seems correct so far, but I am unsure about the exact nature of the error. Your assistance is greatly appreciated.

Here is a screenshot of the error message from the console: https://i.sstatic.net/O37l4.jpg

Data.ts file - showcasing two items from the array:

export const Assignments: Assignment[] = [
  {
    "a_type": "one",
    "a_title": "Assignment 1",
    "symbol": 1,
    "show": false,
    "tooltip": {
        "left": 82
    },
    "buttonPos":{
        "left": 130
    },
    "printTop": 0,
    "instructions": "Instructions here",
    "due_date": "sept-15.png",
    "percentage": "10.png",
    "taskA": {
        "name": "Option A",
        "a_title": "Task A",
        "information": "Instructions for task A",
        "selectA": true
    }
}, {
    "a_type": "two",
    "a_title": "Assignment 2",
    "symbol": 2,
    "show": false,
    "sub_a_title": "Assignment Information",
    "tooltip": {
        "left": 200
    },
    "buttonPos":{
        "left": 250
    },
    "printTop": 250,
    "instructions": "Instructions here",
    "due_date": "29.png",
    "percentage": "10.png",
    "taskA": {
      "a_title": "Assignment 2 info",
        "name": "Option A",
        "information": "Instructions for task A",
        "selectA": false
    },
    "taskB": {
      "a_title": "Assignment 2 info",
        "name": "Option B",
        "information": "Instructions for task B",
        "selectB": false
    }
}
]

Assignment.ts - defining the data types:

export class Assignment {
    a_type: string;
    a_title: string;
    symbol: any;
    show: boolean;
    tooltip: any;
    left: number;
    buttonPos:any;
    printTop: number;
    instructions: string;
    due_date: string;
    percentage: string;
    taskA: any;
    name: string;
    information: string;
    selectA: boolean;
    taskB: any;
    selectB: boolean;
  }

javascript angular typescript angular2-cli

Answer 1

Answer №1

It's important to note that the reason for the issue is due to the mismatch in structure between the object literals and the Assignment structure.

Typescript operates as a structurally typed language, signifying that a class's type and properties are determined by its structure. An object literal can be viewed as a type of class if their structures align. For instance, consider this class definition:

class Person {
  firstName: string;
  lastName: string;
}

Rather than instantiate a class conventionally with the new keyword,

let person: Person = new Person();
person.firstName = "Stack";
person.lastName = "Overflow";

We can alternatively do the following:

let person: Person = {
  firstName: "Stack",
  lastName: "Overflow"
}

If we omit the lastName property, a compile error would occur since the structure does not match that of the Person class that we attempted to assign it as.

In your code, some errors I noticed include:

The absence of name and information because they are nested within typeA. This arrangement is incorrect as they should be within the main structure defined in Assignment.
The necessity of including taskB in the initial object
Failure to incorporate selectA and selectB from the main structure of the objects.

There may be additional errors present, but hopefully, this clarifies the point being made.

To make elements optional, you can utilize the ? operator

interface Assignment {
  name?: string;
}

You can also implement nesting if desired

interface Assignment {
  taskA?: { name: string },
  taskB?: { name: string }
}

Additional Resources:

Answer 2

It's important to note that the reason for the issue is due to the mismatch in structure between the object literals and the Assignment structure.

Typescript operates as a structurally typed language, signifying that a class's type and properties are determined by its structure. An object literal can be viewed as a type of class if their structures align. For instance, consider this class definition:

class Person {
  firstName: string;
  lastName: string;
}

Rather than instantiate a class conventionally with the new keyword,

let person: Person = new Person();
person.firstName = "Stack";
person.lastName = "Overflow";

We can alternatively do the following:

let person: Person = {
  firstName: "Stack",
  lastName: "Overflow"
}

If we omit the lastName property, a compile error would occur since the structure does not match that of the Person class that we attempted to assign it as.

In your code, some errors I noticed include:

The absence of name and information because they are nested within typeA. This arrangement is incorrect as they should be within the main structure defined in Assignment.
The necessity of including taskB in the initial object
Failure to incorporate selectA and selectB from the main structure of the objects.

There may be additional errors present, but hopefully, this clarifies the point being made.

To make elements optional, you can utilize the ? operator

interface Assignment {
  name?: string;
}

You can also implement nesting if desired

interface Assignment {
  taskA?: { name: string },
  taskB?: { name: string }
}

Additional Resources:

Answer 3

Answer №2

It's quite evident what the error is pointing towards:

The property 'left' is missing ...

In your Assignment class, there is a member named left, but this field is absent in both of your JSON objects.
Moreover, there are several other properties that have not been set in your JSON objects. You can see a functioning version in the TypeScript playground (here).

If you wish, you can declare the properties as optional:

class Assignment {
    a_type?: string;
    ...
}

Even without these errors, there is still an underlying issue.
You are not instantiating instances of the Assignment class; rather, you are creating objects that align with the class properties.
While this may work due to TypeScript employing duck typing, it could lead to problems if your Assignment class includes methods that these objects do not possess.
One workaround is to:

export const Assignments: Assignment[] = [new Assignment(), new Assignment()];

Object.assign(Assignments[0], FIRST_JSON_OBJ);
Object.assign(Assignments[1], SECOND_JSON_OBJ);

Alternatively, you can implement a constructor in the Assignment class that takes in these JSON objects and initializes itself accordingly.

Answer 4

It's quite evident what the error is pointing towards:

The property 'left' is missing ...

In your Assignment class, there is a member named left, but this field is absent in both of your JSON objects.
Moreover, there are several other properties that have not been set in your JSON objects. You can see a functioning version in the TypeScript playground (here).

If you wish, you can declare the properties as optional:

class Assignment {
    a_type?: string;
    ...
}

Even without these errors, there is still an underlying issue.
You are not instantiating instances of the Assignment class; rather, you are creating objects that align with the class properties.
While this may work due to TypeScript employing duck typing, it could lead to problems if your Assignment class includes methods that these objects do not possess.
One workaround is to:

export const Assignments: Assignment[] = [new Assignment(), new Assignment()];

Object.assign(Assignments[0], FIRST_JSON_OBJ);
Object.assign(Assignments[1], SECOND_JSON_OBJ);

Alternatively, you can implement a constructor in the Assignment class that takes in these JSON objects and initializes itself accordingly.

TypeScript: does not match type error

Answer №1

Answer №2

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