What about a toggle for read-only TypeScript everywhere? (parameters in functions)

Is there a method, whether through a macro library, an eslint rule, a tsconfig setting, a special global.d.ts file, or some other means, to automatically set function arguments as readonly by default?

// I wish for the compiler to transform this:
function foo(a: A[], b: Record<string, string>) {
}
// Into this
function foo(a: ReadonlyArray<A>, b: Readonly<Record<string, string>>) {
}

Answer №1

Take a look at the no-param-reassign ESLint rule:

The main goal of this rule is to prevent unintended consequences that may arise from altering or assigning new values to function parameters.

It's important to note that this rule doesn't serve as a replacement for using readonly:

/* eslint no-param-reassign: ["error", { "props": true, "ignorePropertyModificationsForRegex": ["^mut"] }] */
function f(n: number, nums: number[], o: { p: string }, mutArr: number[]) {
    n = 10 // will result in an error
    n++ // will result in an error
    for (n in nums) { } // will result in an error
    nums[1] = 10 // will result in an error
    nums.push(1) // will pass
    nums.sort() // will pass
    o.p = 'foo' // will result in an error
    mutArr[0] = 10 // will pass
}

Answer №2

An ongoing discussion in the typescript github repository is centered around a feature request similar to this:

https://github.com/microsoft/TypeScript/issues/62941

The issue of handling untyped library code seems to be a major concern, which may delay the decision on implementing this feature.

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