TypeScript Array Sort Not Working as Expected with Optional Property

I am trying to sort an array of objects in TypeScript based on an optional property ind. If both a and b have the ind property, I want to sort the array in descending order based on ind. Otherwise, if either a or b (or both) lacks the ind property, I want to sort the array in ascending order based on the createdAt property.

Here's my sorting code:

const sortedArray = arrayToSort.sort((a, b) => {
  if (a.ind && b.ind) {
    return b.ind - a.ind; // Sort by ind in descending order
  } else {
    return (
      new Date(a.createdAt).getTime() -
      new Date(b.createdAt).getTime()
    ); // Sort by createdAt in ascending order
  }
});

However, the sorting does not seem to work as expected, and the result is not sorted correctly.

The array elements have the following structure:

interface ArrayElement {
  ind?: number;
  createdAt: string;
  // other properties...
}

From your interface it seems you ind type is number, if ind is 0 it will be considered as false and even if there is ind in that situation that condition wont be satisfied.

Solution is to check for key availability this way.
`

if (a.ind !== undefined && b.ind !== undefined) {
    return b.ind - a.ind;
  } else {
    rest of the code....

OR

if (a.hasOwnProperty('ind') && b.hasOwnProperty('ind')) {
    return b.ind - a.ind;
  } else {
    rest of the code...

As ind property is optional, Typescript can not guarantee that a.ind and b.ind are always defined.

As per your current code, You are directly subtract the ind values when both a.ind and b.ind are available. To get rid from typescript error, You can check for undefined values.

Live Demo :

<!-- begin snippet: js hide: false console: true babel: false -->

<!-- language: lang-js -->

const arrayToSort = [{
  ind: 1
}, {
  ind: 5
}, {
  ind: 8
}, {
  ind: undefined
}, {
  ind: 7
}, {
  ind: 2
}]

const sortedArray = arrayToSort.sort((a, b) => {
  if (a?.ind !== undefined && b?.ind !== undefined) {
    return b.ind - a.ind;
  } else if (a?.ind !== undefined) {
    return -1; // Move 'a' before 'b'
  } else if (b?.ind !== undefined) {
    return 1; // Move 'b' before 'a'
  } else {
    return
      new Date(a.createdAt).getTime() -
      new Date(b.createdAt).getTime()
  }
});

console.log(sortedArray);

<!-- end snippet -->