Is [number, number] a tuple type?

Question

In the below code,

function f([first, second]: [number, number]){
    console.log(first);
    console.log(second);
}

var input:number[] = [1,2];
f(input);

number[] type variable(input) is passed to f.

Why compiler errors out? Argument of type 'number[]' is not assignable to parameter of type '[number, number]'. Type 'number[]' is missing the following properties from type '[number, number]': 0, 1ts(2345)

Answer 1

TypeScript can be very specific in what it expects to be passed to a method.

For your example:

function f([first, second]: [number, number]){
    console.log(first);
    console.log(second);
}

This will work:

var x: [number, number] = [1,2];

f([1,2]); // Type is implied:  [number, number]
f(x);     // Type is explicit: [number, number]

This won't:

var x = [1,2,3];
var y = [1,2];
var z: number[] = [1,2];

f(x); // Type is implied:  number[]
f(y); // Type is implied:  number[]
f(z); // Type is explicit: number[]

You told typescript to expect an array containing 2 numbers. That's why it won't accept anything but an array containing 2 numbers.

You can change the accepted type like this:

function f([first, second]: number[]){
// Keep destructuring ^,      ^ but change the accepted type.
    console.log(first);  
    console.log(second);
}

Then, any of the previous 6 examples will work, as [number, <number...>] are also number[] arrays.