The altered closure variable ts remains undetectable

Question

The altered closure variable ts remains undetectable

I made changes to the flag variable, but TypeScript did not recognize it. Could this be a coding issue?

function fn () {
  let flag = true

  function f () {
    // alter the value of flag
    flag = false
  }

  for (let i = 0; i < 10; i++) {
    // error: This condition will always return 'false' since the types 'true' and 'false' have no overlap.
    if (flag === false) { break }
    f()
  }

  return flag
};

fn()

typescript

Answer 1

Answer №1

For a detailed response to this query, refer to microsoft/TypeScript#9998.

The issue observed is not a bug but rather a TypeScript design limitation that is challenging to address easily. An illustration of functional code:

function fn() {
  let flag = true
  for (let i = 0; i < 10; i++) {
    if (flag === false) { break }
    flag = false;
  }
  return flag
};

In the provided example, the compiler recognizes flag as type boolean, a shorthand notation in TS for the union true | false. The compiler narrows down the apparent type of flag from true | false to just true after the initial assignment of true. However, within the loop scope, the reassignment to false resets the narrowing to

boolean</code inside the loop.</p>
<hr />
<p>Unfortunately, when refactoring the <code>flag = false

line into the body of f(), the compiler does not reset the narrowing post function call execution, resulting in an error due to incorrect assumption about flag's type.

The reason behind this behavior is the limitations in accurately and efficiently performing control flow analysis with closure mutations. Inline expansion for every function call would be impractical, leading to extended compile times, hindering performance.

As accuracy isn't achievable, the compiler relies on optimistic assumptions like assuming function calls have no impact on state. This approach saves compilation time but can lead to errors in scenarios where function calls do indeed alter state.

While there may be room for improvement in this area, at present, TypeScript struggles to track state mutations within function calls.

To prevent such erroneous errors when dealing with state-changing functions, refrain from narrowing flag's type altogether by adjusting its initial assignment as follows:

let flag = true as boolean;

This widens the type of the true value to true | false, keeping flag indeterminate throughout the scope according to the compiler.

Playground link to code

Answer 2

For a detailed response to this query, refer to microsoft/TypeScript#9998.

The issue observed is not a bug but rather a TypeScript design limitation that is challenging to address easily. An illustration of functional code:

function fn() {
  let flag = true
  for (let i = 0; i < 10; i++) {
    if (flag === false) { break }
    flag = false;
  }
  return flag
};

In the provided example, the compiler recognizes flag as type boolean, a shorthand notation in TS for the union true | false. The compiler narrows down the apparent type of flag from true | false to just true after the initial assignment of true. However, within the loop scope, the reassignment to false resets the narrowing to

boolean</code inside the loop.</p>
<hr />
<p>Unfortunately, when refactoring the <code>flag = false

line into the body of f(), the compiler does not reset the narrowing post function call execution, resulting in an error due to incorrect assumption about flag's type.

The reason behind this behavior is the limitations in accurately and efficiently performing control flow analysis with closure mutations. Inline expansion for every function call would be impractical, leading to extended compile times, hindering performance.

As accuracy isn't achievable, the compiler relies on optimistic assumptions like assuming function calls have no impact on state. This approach saves compilation time but can lead to errors in scenarios where function calls do indeed alter state.

While there may be room for improvement in this area, at present, TypeScript struggles to track state mutations within function calls.

To prevent such erroneous errors when dealing with state-changing functions, refrain from narrowing flag's type altogether by adjusting its initial assignment as follows:

let flag = true as boolean;

This widens the type of the true value to true | false, keeping flag indeterminate throughout the scope according to the compiler.

Playground link to code

The altered closure variable ts remains undetectable

Answer №1

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