Flattening Array of Arrays in TypeScript


Working with arrays is an integral part of programming in TypeScript, and one of the most common tasks you might encounter is flattening an array of arrays. This process involves transforming a multidimensional array into a single-dimensional one, which can be a bit tricky if you're not familiar with specific methods. In this guide, we'll cover the basics of array flattening in TypeScript and discuss potential inference issues that can arise. We'll also delve into how to flatten arrays using the forEach() method.

Why flatten arrays?

Array flattening is the process of converting an array of arrays into a single array. In other words, it's about removing the nested structure of a multi-dimensional array.

This might be useful, for example, when you need to process all of the elements in a nested array in a sequential manner.

Consider the following array of arrays:

let arr: number[][] = [[1, 2], [3, 4], [5, 6]];

After flattening, we would end up with a single array:

let flatArr: number[] = [1, 2, 3, 4, 5, 6];

This simplifies the process of iterating over the array, as we no longer have to deal with nested loops.

Potential Inference Issues with Flattened Arrays

When working with TypeScript, you need to be aware of potential inference issues that might occur when flattening arrays. TypeScript is a statically typed language, meaning it checks types at compile time. When you flatten an array, TypeScript needs to then infer the type of the resulting array.

Consider an array of arrays where each subarray contains elements of different types:

let mixedArr: (number | string)[][] = [[1, "a"], [2, "b"], [3, "c"]];

If we were to flatten this array, TypeScript would have to infer the type of the resulting array. In this case, TypeScript correctly infers the type as (number | string)[].

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However, if the types in the subarrays are not consistent or clear, TypeScript might not be able to correctly infer the type. So make sure that the types are consistent to avoid any potential issues.

Flattening Arrays Using forEach() Method

Using the forEach() method to flatten an array is one possible way to do so. This method executes a provided callback function once for each array element. Here's how you can use it to flatten an array:

let arr: number[][] = [[1, 2], [3, 4], [5, 6]];
let flatArr: number[] = [];

arr.forEach(subArr => {
    subArr.forEach(element => {

console.log(flatArr); // Output: [1, 2, 3, 4, 5, 6]

In this code, we first declare a new empty array flatArr. We then use the forEach() method twice: once to iterate over the outer array, and once to iterate over each subarray. For each element in the subarrays, we use the push() method to add it to flatArr. The result is our new flattened array!

Flattening Arrays Using reduce() Method

The reduce() method in TypeScript is another great tool to use to flatten an array of arrays. It applies a function against an "accumulator" and each element in the array (from left to right) to reduce it to a single output value.

Here's an example of how you can use reduce() to flatten an array:

let arr = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];
let flatArr = arr.reduce((acc, value) => acc.concat(value), []);


When you run this code, you should see the following output:

[1, 2, 3, 4, 5, 6, 7, 8, 9]

Here, reduce() starts with an empty array as the accumulator (the second argument to reduce(), []) and then concatenates each sub-array to it. The result is a new flattened array.

Note: The reduce() method does not change the original array. Instead, it returns a new array. This is an important point to remember when working with array methods in TypeScript.


Flattening an array of arrays in TypeScript might seem challenging at first, but with the right methods, it can be fairly simple. The forEach() and reduce() methods offer simple solutions for flattening arrays, each with their own benefits. And remember, these methods do not modify the original array, but instead return a new flattened array.

Last Updated: August 18th, 2023
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