Using Typescript: Undefined arrays could cause issues in the array map function

Question

Using Typescript: Undefined arrays could cause issues in the array map function

Encountering a Typescript error stating that

'<array name>' is possibly undefined

while attempting to map over an array in Typescript. Below is an example code snippet triggering this issue:

type obj = {
    list?: string[]
};

function demonstration(ex: obj) {
    ex.list?.map((str) => {
        ex.list.findIndex(s => s === str); // error here
    })
}

Aware of the workaround involving adding an if check at the beginning of the map function, but keen on understanding the root cause behind this error.

If the list's map function does run, doesn't it imply that the list exists already? Speculation around it being a potential Typescript bug arises due to lack of clarity as a newcomer to Typescript. Appreciate any guidance provided.

typescript

Answer 1

Answer №1

The limitations of TypeScript are well-documented, with a detailed discussion available at microsoft/TypeScript#9998. The current type system lacks the ability to recognize or indicate when a callback is executed, leading to any narrowing of closed-over variables being reset within the scope of such callbacks.

To address this issue, various workarounds exist, with one popular approach being to create a new constant variable in the same scope as the narrowed value and then use this new variable inside the closure. This guarantees that the new variable will always maintain the narrowed type:

function demo(example: Obj) {
  if (example.list) {
    const list = example.list; // copy narrowed value to new variable
    list.map((str) => {
      list.findIndex(s => s === str); // okay
    })
  }
}

For scenarios involving array functional methods like map() and access to the original array, you can modify your callback parameters to include all three arguments, with the third representing the original array:

function demo(example: Obj) {
  example.list?.map((str, _, list) => { // <-- third parameter represents the array
    list.findIndex(s => s === str); // okay
  })
}

Link to Playground for interactive code demonstration

Answer 2

The limitations of TypeScript are well-documented, with a detailed discussion available at microsoft/TypeScript#9998. The current type system lacks the ability to recognize or indicate when a callback is executed, leading to any narrowing of closed-over variables being reset within the scope of such callbacks.

To address this issue, various workarounds exist, with one popular approach being to create a new constant variable in the same scope as the narrowed value and then use this new variable inside the closure. This guarantees that the new variable will always maintain the narrowed type:

function demo(example: Obj) {
  if (example.list) {
    const list = example.list; // copy narrowed value to new variable
    list.map((str) => {
      list.findIndex(s => s === str); // okay
    })
  }
}

For scenarios involving array functional methods like map() and access to the original array, you can modify your callback parameters to include all three arguments, with the third representing the original array:

function demo(example: Obj) {
  example.list?.map((str, _, list) => { // <-- third parameter represents the array
    list.findIndex(s => s === str); // okay
  })
}

Link to Playground for interactive code demonstration

Answer 3

Answer №2

The behavior described above is a result of TypeScript's type checker only functioning at the highest level. When you introduce another variable that points to ex.list, TypeScript can correctly infer the type. However, this inference does not extend to properties of an object:

type customObj = {
    list?: string[]
};

function worksFine(ex: customObj) {
    const e = ex.list
    if (e)
      e.map((str) => {
        e.findIndex(s => s === str); // no error as the type of e is known to be an array
      })
}

function alsoWorks(ex: customObj) {
  ex.list?.map((str) => {
    ex.list?.findIndex(s => s === str); // explicit checking ensures no errors
  })
}

function hasError(ex: customObj) {
  ex.list?.map((str) => {
    ex.list.findIndex(s => s === str); // error will occur here
  })
}

You can try out the code on TypeScript's playground example.

Answer 4

The behavior described above is a result of TypeScript's type checker only functioning at the highest level. When you introduce another variable that points to ex.list, TypeScript can correctly infer the type. However, this inference does not extend to properties of an object:

type customObj = {
    list?: string[]
};

function worksFine(ex: customObj) {
    const e = ex.list
    if (e)
      e.map((str) => {
        e.findIndex(s => s === str); // no error as the type of e is known to be an array
      })
}

function alsoWorks(ex: customObj) {
  ex.list?.map((str) => {
    ex.list?.findIndex(s => s === str); // explicit checking ensures no errors
  })
}

function hasError(ex: customObj) {
  ex.list?.map((str) => {
    ex.list.findIndex(s => s === str); // error will occur here
  })
}

You can try out the code on TypeScript's playground example.

Answer 5

Answer №3

To begin with, it is essential that the callback function for your map method actually returns a value, otherwise you will end up with an array filled with undefined elements.

Additionally, while type-inference can be helpful, it does have its limitations. If you are confident that the property ex.list exists in the current context even though it is defined as list?: string[] (and in this particular scenario it must exist for the callback to be invoked), you can utilize the non-null assertion operator explained here.

type obj = {
    list?: string[]
};

function exampleFunction(ex: obj) {
    ex.list?.map((str) => {
        return ex.list!.findIndex(s => s === str); // there's an error here
    })
}

Answer 6

To begin with, it is essential that the callback function for your map method actually returns a value, otherwise you will end up with an array filled with undefined elements.

Additionally, while type-inference can be helpful, it does have its limitations. If you are confident that the property ex.list exists in the current context even though it is defined as list?: string[] (and in this particular scenario it must exist for the callback to be invoked), you can utilize the non-null assertion operator explained here.

type obj = {
    list?: string[]
};

function exampleFunction(ex: obj) {
    ex.list?.map((str) => {
        return ex.list!.findIndex(s => s === str); // there's an error here
    })
}

Using Typescript: Undefined arrays could cause issues in the array map function

Answer №1

Answer №2

Answer №3

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