How can I define Array.reduce method in Typescript?

Question

I have a problem with the reduce method in TypeScript:

const items = {a: 10, b:20, c: 30}
const itemsTotal = Object.keys(items).reduce((accumulator: number, key: keyof typeof items ) => {
    return accumulator + items[key]
  }, 0)

I keep receiving the Typescript error:

Argument of type '(accumulator: number, key: "a" | "b" | "c") => number' is not assignable to parameter of type '(previousValue: string, currentValue: string, currentIndex: number, array: string[]) => string'.

 Types of parameters 'accumulator' and 'previousValue' are incompatible.***

It seems that I need to define the type of the reduce method, but how?

Answer 1

Object.keys returns a string[] , so you can't apply a reducer function that expects a keyof typeof items to it.

You could use a type assertion, since you know it's valid:

const items = {a: 10, b:20, c: 30};
const itemsTotal = Object.keys(items).reduce((accumulator, key) => {
    return accumulator + items[key as keyof typeof items];
}, 0);

Playground

...but you don't need the key anyway, just use Object.values :

const items = {a: 10, b:20, c: 30};
const itemsTotal = Object.values(items).reduce((accumulator, value) => {
    return accumulator + value;
}, 0);

Playground

(But frankly I'd just use a simple loop .)

Answer 2

Define your items as a Record of string to number . It will be clear for the reduce method that the item is a number .

const items: Record<string, number> = {a: 10, b: 20, c: 30}

const itemsTotal = Object.keys(items).reduce((accumulator: number, key: string) => {
    return accumulator + items[key];
}, 0)

You can also skip the braces in the reduce body.

Object.keys(items).reduce((acc: number, key: string) => acc + items[key], 0)

Even more, you can skip the type designations in the reduce because your items are already defined as numbers in the Record .

Object.keys(items).reduce((acc, key) => acc + items[key], 0)

Answer 3

Not the cleanest solution but you can use this snippet:

const items = { a: 10, b: 20, c: 30 }
const itemsTotal = Object.keys(items).reduce((accumulator, key) => {
    return accumulator + items[key as keyof typeof items]
}, 0)

Key point is to cast key to keyof typeof items