TypeScript cannot infer array type in spread syntax function call

Question

I can spread an array as function/constructor arguments without any problem: (test.js)

class Person {
    name; 
    age;

    constructor(name, age) {
        this.name = name;
        this.age = age;
    }
}

let a = ['Claire', 8];
let p = new Person(...a);

console.log(p);

However, the same thing doesn't work in TypeScript: (test.ts)

class Person {
    constructor(
        public name: string,
        public age: number
    ) {}
}

let a = ['Claire', 8];
let p = new Person(...a);  // Error: An argument for 'name' was not provided.

console.log(p);

Is this a bug in TypeScript? It cannot figure out how to use spread syntax here. Is there a workaround or I have to manually assign each individual argument?

Dimitar Nestorov · Accepted Answer · 2020-04-15 19:58:13Z

4

Option 1:

let a: [string, number] = ['Claire', 8];

Option 2:

let a = ['Claire', 8] as const;

This will tell TypeScript to treat a as a tuple containing 'Claire' as its first item and 8 as its second which conforms to [string, number]. Without as const a was treated as a (string | number)[].

edited Apr 15, 2020 at 19:58

answered Apr 15, 2020 at 18:20

Dimitar Nestorov

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3 Comments

bryan60 Over a year ago

is this different from declaring it as const a = ['Claire', 8]

Dimitar Nestorov Over a year ago

@bryan60 the JavaScript result will be the same, TypeScript will treat it differently

Alexandre Fradette Over a year ago

@bryan60 const a = in that case will just tell that you won't reassign any value to this variable, it doesn't affect the type.

Rui Wang · Accepted Answer · 2020-04-15 19:43:17Z

0

Thank you so much! Now I understand the error is due to type incompatibility. The following code works (test.ts)

class Person {
    constructor(
        public name: string,
        public age: number
    ) {}
}

/* Spell out type or use 'as const' both work!
Or TypeScript treats ['Claire', 8] as of type (string | number)[]*/
let a: [string, number] = ['Claire', 8];
let b = ['Liam', 11] as const;

let p1 = new Person(...a);
let p2 = new Person(...b);

console.log(p1, p2);

answered Apr 15, 2020 at 19:43

Rui Wang

1911 silver badge5 bronze badges

1 Comment

josemigallas Over a year ago

Please mark the original answer as correct if it led you to a solution.

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