Type 'Number' cannot be used as an index type in TypeScript

Type 'Number' cannot be used as an index type in TypeScript #

The "Type 'Number' cannot be used as an index type" TypeScript error occurs when we use the Number type instead of number (lowercase n). To solve the error, make sure to only use the number type with lowercase n in your TypeScript code.

Here is an example of how the error occurs.

Copied!
const arr1: Number[] = [0, 1, 2]; // 👈️ capital N

const arr2 = ['a', 'b', 'c'];

// ⛔️ Type 'Number' cannot be used as an index type.ts(2538)
arr2[arr1[0]];

We used the Number, non-primitive object type to type the arr1 variable, which caused the error.

To solve this, always use the number type when typing numbers in TypeScript.

Copied!
// ✅ now using number
const arr1: number[] = [0, 1, 2];

const arr2 = ['a', 'b', 'c'];

console.log(arr2[arr1[0]]); // 👉️ "a"

We used the primitive number type which resolved the error.

The error was caused because there is a difference between the primitive number, string and boolean types and the non-primitive Number, String, Boolean, Object, etc.

The non-primitive types are objects and should never be used when typing values in TypeScript.

The TypeScript best practices documentation warns to never use the Number, String, Boolean, Symbol or Object non-primitive objects when typing values in your TypeScript code.

Instead, you should be using number, string, boolean, symbol and Record types.

Copied!
const num: Number = 1;

const arr = ['a', 'b', 'c'];

// ⛔️ Error: Type 'Number' cannot be used as an index type.ts(2538)
arr[num]

const obj = {
  0: 'a',
  1: 'b',
  2: 'c',
};


const num2: Number = 2;

// ⛔️ Error: Type 'Number' cannot be used as an index type.ts(2538)
obj[num2]

The solution is to use the number type or let TypeScript infer the type if declaring the value inline.

Copied!
const num = 1; // 👈️ type is inferred

const arr = ['a', 'b', 'c'];

console.log(arr[num]); // 👉️ "b"

const obj = {
  0: 'a',
  1: 'b',
  2: 'c',
};

const num2 = 2; // 👈️ type is inferred

console.log(obj[num2]); // 👉️ "c"

Conclusion #

The "Type 'Number' cannot be used as an index type" TypeScript error occurs when we use the Number type instead of number (lowercase n). To solve the error, make sure to only use the number type with lowercase n in your TypeScript code.