Rule suggestion: disallow modifying function parameters

[benny-medflyt]

There is an argument that using const instead of let makes code more readable, since you can be sure at a glance that a variable will not change it's value. (See prefer-const)

Unfortunately, in TypeScript (or JavaScript) there is no way to declare that a particular function parameter should be const (they are all implicitly let).

For example, the following function modifies the x parameter:

function add(x: number, y: number): number {
    x += y;
    return x;
}

I imagine that a lot of projects would be happy to completely ban all such usages in their code base: all function parameters would behave as if they were const. I believe that this can be implemented as a tslint rule, and I suggest that such a rule be added =]

[ajafff]

IMO such a rule makes sense. The implementation should not be too different from prefer-const. Maybe we can extract some of the logic into a reusable class or function.
Until it is implemented in tslint core, you can use a third party rule: https://github.com/vrsource/vrsource-tslint-rules#no-param-reassign

Just one question if we want to treat destructured parameters as real parameters? Would be best to make it configurable, I guess.

function foo(bar: number) {} // this is a parameter
function foo([bar, baz]: number[]) {} // technically these are local variables ... do we want to treat them as parameter?