Typescript Map class constructor with initial values not accepting 2 different types

Question

First things first, I'm using these classes:

class Student {
  name: string;
  age: number;
  constructor(name: string, age: number) {
    this.name = name;
    this.age = age;
  }
}

class Food {
  flavor: string;
  portions: number;
  constructor(flavor: string, portions: number) {
    this.flavor = flavor;
    this.portions = portions;
  }
}

Basically what I'm doing is that:

const food_backpack = new Map<Student, Food>()

const sam = new Student('Sam', 15);
const ariana = new Student('Ariana', 18);

const cheese = new Food('Fetta', 5);
const chocolate = new Food('Twix', 2);

food_backpack.set(sam, cheese);
food_backpack.set(ariana, chocolate);

Well that worked.

But I'm trying to use the constructor instead to initialize the map values which IS NOT WORKING for me (compile time errors). I tried this below:

const sam = new Student('Sam', 15);
const ariana = new Student('Ariana', 18);

const cheese = new Food('Fetta', 5);
const chocolate = new Food('Twix', 2);

const bi_sam = [sam, cheese];
const bi_ariana = [ariana , chocolate];

const food_backpack = new Map<Student, Food>([
  bi_sam,
  bi_ariana
]);

And this below:

const sam = new Student('Sam', 15);
const ariana = new Student('Ariana', 18);

const cheese = new Food('Fetta', 5);
const chocolate = new Food('Twix', 2);

const bi_sam = [(sam as Student) , (cheese as Food)];
const bi_ariana = [(ariana as Student) , (chocolate as Food)];

const food_backpack = new Map<Student | Food, Food | Student>([
  bi_sam,
  bi_ariana
]);

Something that uses the constructor way and THAT WORKED is:

const sam = new Student('Sam', 15);
const ariana = new Student('Ariana', 18);

const cheese = new Food('Fetta', 5);
const chocolate = new Food('Twix', 2);

const food_backpack = new Map<Student, Food>([
  [sam, cheese], 
  [ariana, chocolate]
]);

But I don't prefer it.

Thanks for your precious time and effort!

Argument of type '(Student | Food)[][]' is not assignable to parameter of type 'ReadonlyArray<[Student | Food, Student | Food]>'. Type '(Student | Food)[]' is missing the following properties from type '[Student | Food, Student | Food]': 0, 1 — Jhon
– Jhon, Commented Aug 21, 2020 at 21:44
Without a context, like an argument list, to guide inference array types are inferred instead of tuple types. — Aluan Haddad
– Aluan Haddad, Commented Aug 21, 2020 at 22:34

butttons · Accepted Answer · 2020-08-21 22:03:34Z

4

TypeScript is unable to match the signature you provided and the one that is passed by the arguments. Mainly, the values are readonly in a Map.

You can create a new type like,

type StudentRecord = readonly [Student, Food];

And now your Map constructor should work as intended:


const sam = new Student('Sam', 15);
const ariana = new Student('Ariana', 18);

const cheese = new Food('Fetta', 5);
const chocolate = new Food('Twix', 2);

type StudentRecord = readonly [Student, Food];

const bi_sam: StudentRecord = [sam, cheese];
const bi_ariana: StudentRecord = [ariana , chocolate];

const food_backpack = new Map<Student, Food>([
  bi_sam,
  bi_ariana
]);

answered Aug 21, 2020 at 22:03

butttons

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5 Comments

Jhon Over a year ago

that I like. Thankss alott!! Btw I saw those weird constructs you’re suggesting but didn’t understand there real purpose until now. I’ll test tomorrow and let you know if all goes to plan.

butttons Over a year ago

Sure thing, you can learn more about how generics work in the official handbook.

Aluan Haddad Over a year ago

You can also just write const bi_sam = [same, cheese] as const;

Jhon Over a year ago

I tried the 'StudentRecord' and the 'as const' solutions and I still get compile time errors. I'm working on repl.it/languages/typescript . Is this compiler outdated?

butttons Over a year ago

Yes, that appears to be running v3.3.3. readonly Tuples were added in v.3.4. I'd recommend the official playground available, it has versions you can switch too.

Konrad · Accepted Answer · 2020-08-21 22:02:26Z

-1

As we can see at Map mdn page it takes arguments like this:

new Map([
  [1, 'one'],
  [2, 'two'],
  [3, 'three'],
])

So it's an array of two-element array. First element is a key and second a value. So it perfectly explain why it works with:

new Map<Student, Food>([
  [sam, cheese], 
  [ariana, chocolate]
])

but now with:

new Map<Student, Food>([
  bi_sam,
  bi_ariana
])

answered Aug 21, 2020 at 22:02

Konrad

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1 Comment

Jhon Over a year ago

I read that page and that’s what puzzles me. My bi_sam and bi_ariana are just variables representing the exact same thing. I’m starting to get a feel that I’m creating an array of array pointers instead of array of bi-value arrays. What’s your thoughts on that? And why typescript won’t work it out on its own?

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