How do I get the index of object in array using angular?

An easy and elegant solution is to use _.isEqual, which performs a deep comparison using lodash library

Npm Package -- https://www.npmjs.com/package/lodash

const a = [
   { firstName: "Adam", LastName: "Howard" },
   { firstName: "Ben", LastName: "Skeet" },
   { firstName: "Joseph", LastName: "Skeet" }
];
const find =  { firstName: "Joseph", LastName: "Skeet" }

index = a.findIndex(x => _.isEqual(x, find));

console.log(index);

As mdn says:

The findIndex() method returns the index of the first element in the array that satisfies the provided testing function. Otherwise, it returns -1, indicating that no element passed the test.

If you have array of objects like this, then try to use findIndex:

const a = [
    { firstName: "Adam", LastName: "Howard" },
    { firstName: "Ben", LastName: "Skeet" },
    { firstName: "Joseph", LastName: "Skeet" }
];

let index = a.findIndex(x => x.LastName === "Skeet");
console.log(index);

Adding to the answer of @StepUp (and please select his answer as best, as it replies the question greatly):

Finding the index is not related to Angular, it is rather Vanilla JavaScript.

In your case, you need a function that has as parameter the newDatum "needle" to be searched in the array "haystack" using findIndex:

var getNewDatumIndex = (array, newDatumNeedle) => {
  return array.findIndex(x => x.newDatum === newDatumNeedle); 
}

You can also add the method in the array prototype if you want, in order to ommit the first argument:

Array.prototype.getNewDatumIndex = (newDatumNeedle) => {
  return this.findIndex(x => x.newDatum === newDatumNeedle); 
}