Typescript

Readonly

Typescript supports the readonly attribute. Assume the following function to sort arrays:

function sortNumbers(array: Array<number>) {
    return array.sort((a, b) => a - b)
}

const numbers = [7, 3, 5]

const sortedNumbers = sortNumbers(numbers)

console.log(sortedNumbers)
console.log(numbers)

We can see that the function sortNumbers changed both arrays. We can add the Readonly type, but this alone won’t work. The array still needs to be changed in place and this will generate an error.

function sortNumbers(array: Readonly<Array<number>>) {
    return array.sort((a, b) => a - b)
}

By doing a defensive copy of the array, this will work:

function sortNumbers(array: Readonly<Array<number>>) {
    return [...array].sort((a, b) => a - b)
}