Why does the let keyword trigger an error in this scenario?

interface Incrementor {
  (x: number): number;
  increment: number;
}

const a: Incrementor = function (x) { return 111 };
a.increment = 111;

let a1: Incrementor = function (x) { return 111 };
a1.increment = 111;

When using let, an error message appears saying "Property 'increment' is missing in type '(x: number) => number' but required in type 'Incrementor'". However, this error message does not appear when using const.

What's the difference between using let and const in this scenario?

You can view the original code here: https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAgJIgVCBbC4D2UyA3gFDLIAUAHgFzIgCu2ARtAJT1OvQDc5yUJhx4wXZmyj8AvqVIJ8IAM5hkceumG4CRALzIYjDGGCKq1diWRYwjKCGQBGZ8mn84AOiFZtq-c8d+UgAbCFU4Rw0MH1FCZH1DY1MHGktiazC7BwDXd0cvaJFweKdnXiA

Answer №1

The usage of let and const is incorrect in this case. Both boo and foo should display errors because they are being initialized with a function instead of an Incrementor.

const boo: Incrementor = function (x) { return 111 }; // <- Error
let foo: Incrementor = function (x) { return 111 }; // <- Error

Answer №2

There appears to be a glitch, and I am unsure how Typescript has affected the const variable. Fortunately, I have discovered a solution to fix this issue in the following scenario:

let a1: Incrementor = function(x) { return 11 } as Incrementor;

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