The utilization of the rest parameter in combination with generics

I encountered an issue with my iteration.

The error message "Operator '+=' cannot be applied to types 'number' and 'T'" is showing up.

I am puzzled as to why this is happening.

let a: number = 1, b: number = 2, c: number = 3, d: number = 4;
function sumNumbers<T>(...numbers: T[]): T {
  let total: number = 0;

  for (let num of numbers) {
    total += num;
  }
  return total;
}

console.log(a + `  + ` + b + ` = ` + sumNumbers<number>(a, b));
// 1 + 2 = 3
console.log(a + `  + ` + b + `  + ` + c + `  = ` + sumNumbers<number>(a, b, c));
// 1 + 2 + 3 = 6
console.log(a + `  + ` + b + `  + ` + c + `  + ` + d + `  = ` + sumNumbers<number>(a, b, c, d));
// 1 + 2 + 3 + 4 = 10

Thank you for your assistance.

Answer №1

The error you are experiencing is due to the fact that the + operator is only allowed for two primary types - string and number. In your specific case, it seems like you intend to work with numbers exclusively (as indicated by the local variable s being of type number). I suggest removing the generic type as this function is designed to operate with numbers:

function sum(...numbers: number[]): number {
  let s: number = 0;

  for (let number of numbers) {
    s += number;
  }
  return s;
}

If you prefer to keep the generic type (although there seems to be no clear reason for it), do not use it in the return type declaration since the return value should be the sum of all numbers provided as arguments:

function sum<T extends number>(...numbers: T[]): number {
  let s: number = 0;

  for (let number of numbers) {
    s += number;
  }
  return s;
}

It is important to note that you cannot have a return type of T because setting T = 1 would imply that the return value is also equal to 1. Instead, the return value should be the sum of all instances of 1 within the array of numbers passed as arguments. Thus, performing arithmetic operations on types like T * arr['length'] is not feasible in TypeScript.

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