Suggestion: document and enforce side effects of functions

[evelant]

Allow programmers to more descriptively document the side effects of functions and have Typescript check against these descriptions. Google Closure seems to use this type of documentation for optimization. It could result in better self documenting code, more helpful tooling, and more logic errors caught.

I am fairly new to TypeScript so I'm not sure if this would be really helpful in practice or not. What do you think?

No side effects

/** @nosideeffects */
function a(){
  let foo = "asdf";
  foo = "bar";
  return a
} //OK, only local variables are assigned to

/** @nosideeffects */
function b(){
  return SOME_GLOBAL;
} //OK, reads global state but does not modify it

/** @nosideeffects */
function b(){
  SOME_GLOBAL = "asdf";
} //ERROR, non local variable assigned to

/** @nosideeffects */
function c(bar: string){
  bar = "123"
  return bar
} //OK, non-reference type argument modified

/** @nosideeffects */
function d(bar: {a: string}){
  bar.a = "123"
} //ERROR, reference type argument modified

/** @nosideeffects */
function e(){
  const foo = a();
  return foo
} //OK, only @nosideeffects functions are called


function f(){
  return "foo"
}

/** @nosideeffects */
function g(){
  const foo = f();
  return foo
} //ERROR, cannot call a normal function from a @nosideeffects function

Modifies

function a(){
  let myObj = {foo: {bar: "bar"}}
  b(myObj)
  console.log(foo.bar) //WARN: myObj may have been clobbered by b(myObj)
}

/** @modifies {arg} */
function b(arg: any){
  delete arg.foo
}

[ghost]

Looks like a duplicate of #17181

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.