The element at index '0' is not defined for the data type 'number | [number, number]'

In my current project, I have a component named ComponentA which has a defined interface. Here is the snippet of the interface:

interface A1 {
    isSingle: true;
    value: number;
}

interface A2 {
    isSingle: false;
    value: [number, number];
}

export type IA = A1 | A2;

function ComponentA({ isSingle = false, value }: IA): void {
    if (!isSingle) {
        console.log(value[0]);
    }
};

export default ComponentA;

When passing isSingle=false, we should expect the value as an array with two numeric elements. However, when trying to access value[0], I encountered a TypeScript error:

 Element implicitly has an 'any' type because expression of type '0' can't be used to index type 'number | [number, number]'.   Property '0' does not exist on type 'number | [number, number]'.

I am currently stuck and unable to find a solution for this issue. Any help would be greatly appreciated.

typescript

Answer №1

It is recommended to remove the default function parameter that is currently set on isSingle. By doing so, TypeScript's type inference will not be affected and based on your interface definitions, isSingle should always have a defined value.

interface Type1 {
    isSingle: true;
    value: number;
}

interface Type2 {
    isSingle: false;
    value: [number, number];
}

export type DataType = Type1 | Type2;

function processData({ isSingle, value }: DataType): void {
    if (!isSingle) {
        console.log(value[0]);
    }
};

export default processData;

Answer №2

When dealing with type IA, it combines two distinct types A1 and A2, making it challenging for the compiler to determine whether the value is an array or a number during compile time. It is important to specify the type of value as either numbers or [number, number] before using it.

For Example:


interface A1 {
    isSingle: true;
    value: number;
}

interface A2 {
    isSingle: false;
    value: [number, number];
}

export type IA = A1 | A2;

// Implementing a type guard
const isA2 = (a: IA): a is A2 => !a.isSingle;

function A({ isSingle = false, value }: IA): void {
  if (!isSingle && isA2({ isSingle, value })) {
    console.log(value[0]);
  } else {
    console.log(value);
  }
};

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