Typescript – Type ‘[][]’ is not assignable to type ‘[]’ error

Typescript – Type ‘[][]’ is not assignable to type ‘[]’ error

If you are working with TypeScript, you might have come across the error message “Type ‘[][]’ is not assignable to type ‘[]’”. This error occurs when you try to assign an array of arrays to a variable that expects a single-dimensional array. In this blog post, we will explore the possible solutions to this problem.

Solution 1: Flatten the array

One way to resolve this error is to flatten the array of arrays into a single-dimensional array. This can be achieved using the Array.prototype.flat() method.

    const nestedArray: number[][] = [[1, 2], [3, 4], [5, 6]];
    const flattenedArray: number[] = nestedArray.flat();
    
    console.log(flattenedArray); // Output: [1, 2, 3, 4, 5, 6]
  

By flattening the array, we convert it into a single-dimensional array, which can be assigned to a variable expecting such an array.

Solution 2: Use type assertion

Another solution is to use type assertion to explicitly tell TypeScript the type of the variable. This can be done using the angle bracket syntax () or the “as” keyword.

    const nestedArray: number[][] = [[1, 2], [3, 4], [5, 6]];
    const flattenedArray: number[] = nestedArray as number[];
    
    console.log(flattenedArray); // Output: [[1, 2], [3, 4], [5, 6]]
  

With type assertion, we inform TypeScript that even though the variable is declared as an array of arrays, we want to treat it as a single-dimensional array.

Conclusion

The “Type ‘[][]’ is not assignable to type ‘[]’” error in TypeScript occurs when you try to assign an array of arrays to a variable expecting a single-dimensional array. To resolve this error, you can either flatten the array using the Array.prototype.flat() method or use type assertion to explicitly specify the type of the variable.

We hope this blog post has helped you understand and resolve the error. If you have any further questions or suggestions, feel free to leave a comment below.