Unlocking the Power of Readonly Arrays in TypeScript

Working With Readonly Arrays in TypeScript

The TypeScript language allows developers to define arrays that are read-only, meaning they cannot be changed after they are set. This is an important concept to understand when writing code as it can help ensure that code stays maintainable and reliable over time.

Arrays defined as readonly will throw a compilation error if you try and change them. This makes sure that the values in the array stay consistent and reliable, and you can be sure that the array won’t be unexpectedly changed. It also helps to protect against accidental mutation of the array, as write operations will not be allowed.

Readonly arrays can be easily declared using the readonly keyword before the array type definition. For example, let’s say you’re writing an application in TypeScript with an array of user data. You could declare this array as readonly as shown here:

    const users: readonly string[] = [
        'Bob',
        'Mary',
        'John'
    ]

The readonly keyword is applied to the array syntax to prevent any changes to the array. Any attempts to add, remove, or modify the elements of the array will result in a compilation error.

You can still iterate through the array and use the individual elements, but you won’t be able to modify it in any way.

In some cases, you may need to make changes to the array, such as adding new elements. In those cases, you can use the reset operator to reset the array to a new value. This won’t rewrite existing elements in the array, but it will replace the entire array.

users = [
    ...users,
    'Jane'
]

The spread operator can be used to expand the existing array and add one or more new elements. This is the only way to change a readonly array.

The readonly keyword is an excellent way to make sure the values in your arrays stay consistent and reliable. It can help you avoid unexpected mutations or side effects that could break your applications over time.