Why does this map declaration throw a type error?

Question

I tried declaring a variable where each key in the map is an array of objects. for some reason the act of making the value an array throws a type error.

this is allowed:

var map = new Map([
            ['a', {c: 1}],
            ['b', {c: 1, d: 1}]
        ])

this is not allowed:

var map = new Map([
            ['a', [{c: 1}]],
            ['b', [{c: 1, d: 1}]]
        ])

the second piece of code will throw this error: Type '{ a: number; b: number; }' is not assignable to type '{ a: number; }'

why is the first piece of code allowed but the second not allowed? i would expect to be allowed to have different types on each key of my map

Answer 1

Your types are't consistent. You can explicitly tell Map the types it's going to have

interface MapValue {
  c: number;
  d?: number;
}

const map = new Map<string, MapValue[]>([
  ['a', [{ c: 1 }]],
  ['b', [{ c: 1, d: 1 }]]
])

// or

interface MapValue {
  c: number;
  d: number;
}

const map = new Map<string, MapValue[]>([
  ['a', [{ c: 1 } as MapValue]],
  ['b', [{ c: 1, d: 1}]]