The TypeScript compiler does not allow a 'number' type to be assigned to 0, 10, or 20, even when the number itself is 0

Question

The TypeScript compiler does not allow a 'number' type to be assigned to 0, 10, or 20, even when the number itself is 0

When testing out my code snippet on the playground for Typescript, an error appears on line 24.

I discovered that the issue can be resolved by explicitly casting commands back to <IPlan[]>, but I wonder why this extra step is necessary.

Property 'commands' in type 'Thing' cannot be assigned to the same property in base type 'AbstractThing'.
  Type '{ plan: number; }[]' cannot be assigned to type 'IPlan[]'.
    Type '{ plan: number; }' cannot be assigned to type 'IPlan'.
      The types of the 'plan' property are incompatible.
        Type 'number' cannot be assigned to type '0 | 10 | 20'.
(property) Thing.commands: { plan: number; }[]

Below is the code snippet:

interface IPlan {
  plan: 0 | 10 | 20;
}

// This works
let obj: IPlan = {
  plan: 0
};

// Also works
let commands: IPlan[] = [
  {
    plan: 10
  }
];

// Does not work
abstract class AbstractThing {
  commands?: IPlan[];
}

class Thing extends AbstractThing {
  // The error occurs on this line
  commands = [
    {
      plan: 10
    }
  ]
}

javascript typescript

Answer 1

Answer №1

An identified issue exists where properties and methods are not contextually typed based on the properties of a base class or implemented interface until after normal type inference occurs. This leads to errors in code implementations that should match these constraints.

class Foo {
  x = "hello";
}

interface Interface {
  x: "hello" | "goodbye";
}

class Bar implements Interface {
  x = "hello"; // error!
}

class Base {
  x: "hello" | "goodbye" = "hello";
}
class Baz extends Base {
  x = "hello"; // error!
}

The property x is inferred as a widened string type, causing conflicts with interfaces or classes it should adhere to. Despite previous attempts to resolve this issue, it persists due to complications arising from proposed changes.

To work around this, explicit type annotations can be used to align properties with expected types:

class Bar implements Interface {
  x: Interface['x'] = "hello"; // okay, "hello" | "goodbye"
}

class Baz extends Base {
  readonly x = "hello"; // okay, readonly narrows to "hello" only
}

These annotations help the compiler understand and enforce correct property typings, providing a temporary solution to the problem until a more permanent fix is implemented.

Hopefully these suggestions prove helpful in addressing the issue at hand. Best of luck!

Answer 2

An identified issue exists where properties and methods are not contextually typed based on the properties of a base class or implemented interface until after normal type inference occurs. This leads to errors in code implementations that should match these constraints.

class Foo {
  x = "hello";
}

interface Interface {
  x: "hello" | "goodbye";
}

class Bar implements Interface {
  x = "hello"; // error!
}

class Base {
  x: "hello" | "goodbye" = "hello";
}
class Baz extends Base {
  x = "hello"; // error!
}

The property x is inferred as a widened string type, causing conflicts with interfaces or classes it should adhere to. Despite previous attempts to resolve this issue, it persists due to complications arising from proposed changes.

To work around this, explicit type annotations can be used to align properties with expected types:

class Bar implements Interface {
  x: Interface['x'] = "hello"; // okay, "hello" | "goodbye"
}

class Baz extends Base {
  readonly x = "hello"; // okay, readonly narrows to "hello" only
}

These annotations help the compiler understand and enforce correct property typings, providing a temporary solution to the problem until a more permanent fix is implemented.

Hopefully these suggestions prove helpful in addressing the issue at hand. Best of luck!

Answer 3

Answer №2

It is strange how using the abstract class can cause it to interpret the definition of plan as number rather than the specific values you provided.

To ensure the correct type is utilized, consider implementing one of these "workarounds":

class Thing extends AbstractThing {
  commands = [
    {
      plan: 10
    }
  ] as IPlan[]
}

or

class Thing extends AbstractThing {
  commands: IPlan[] = [
    {
      plan: 10
    }
  ]
}

Answer 4

It is strange how using the abstract class can cause it to interpret the definition of plan as number rather than the specific values you provided.

To ensure the correct type is utilized, consider implementing one of these "workarounds":

class Thing extends AbstractThing {
  commands = [
    {
      plan: 10
    }
  ] as IPlan[]
}

or

class Thing extends AbstractThing {
  commands: IPlan[] = [
    {
      plan: 10
    }
  ]
}

Answer 5

Answer №3

According to my analysis, TypeScript might struggle with inferring the type if the variable is declared outside of a method.

To resolve this issue, simply relocate the declaration inside the constructor, as illustrated here:

class Thing extends AbstractThing {
  constructor() {
    super();
    this.commands = [
      {
        plan: 10
      }
    ];
  }
}

This issue could potentially be just a glitch in the system.

Answer 6

According to my analysis, TypeScript might struggle with inferring the type if the variable is declared outside of a method.

To resolve this issue, simply relocate the declaration inside the constructor, as illustrated here:

class Thing extends AbstractThing {
  constructor() {
    super();
    this.commands = [
      {
        plan: 10
      }
    ];
  }
}

This issue could potentially be just a glitch in the system.

Answer 7

Answer №4

In the definition of your interface, you limit the type of the plan property to be either 0, 10, or 20. However, when you assign the commands property within the class:

commands = [
  {
    plan: 10
  }
]

The plan property is assigned a value of number. Despite setting it to 10, there is no constraint on the type itself, making it less strict than the defined type in the interface, leading to a compiler error.

To resolve this issue, one simple solution is to cast the object to the interface type:

commands = [
  (<IPlan>{
    plan: 10
  })
]

When dealing with types, think of them as sets. If you were the type-checker program and encountered the literal expression

{
  plan: 10,
}

You could interpret this in multiple ways. Here are two possibilities for our discussion:

An object with a plan attribute where the only accepted value is 10.
An object with a plan attribute that can hold any number.

There are additional complexities to consider (like null values or other keys), but the compiler may struggle to infer compatibility solely based on using the literal '10'. Additionally, the value might change dynamically at runtime, adding to the uncertainty.

Answer 8

In the definition of your interface, you limit the type of the plan property to be either 0, 10, or 20. However, when you assign the commands property within the class:

commands = [
  {
    plan: 10
  }
]

The plan property is assigned a value of number. Despite setting it to 10, there is no constraint on the type itself, making it less strict than the defined type in the interface, leading to a compiler error.

To resolve this issue, one simple solution is to cast the object to the interface type:

commands = [
  (<IPlan>{
    plan: 10
  })
]

When dealing with types, think of them as sets. If you were the type-checker program and encountered the literal expression

{
  plan: 10,
}

You could interpret this in multiple ways. Here are two possibilities for our discussion:

An object with a plan attribute where the only accepted value is 10.
An object with a plan attribute that can hold any number.

There are additional complexities to consider (like null values or other keys), but the compiler may struggle to infer compatibility solely based on using the literal '10'. Additionally, the value might change dynamically at runtime, adding to the uncertainty.

The TypeScript compiler does not allow a 'number' type to be assigned to 0, 10, or 20, even when the number itself is 0

Answer №1

Answer №2

Answer №3

Answer №4

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