The findIndex receives a function (predicate) that accepts three arguments and returns a value that is coercible to the Boolean value true or false.

The predicate returns true. If such an element is found, findIndex immediately returns the index of the first element value. Otherwise, findIndex returns -1, indicating that no element passed the test.

const array = [
    {name: 'Freddy', age: 10}, 
    {name: 'Francisco', age: 3}, 
    {name: 'Emilia', age: 4}
];

const preschoolerIndex = array.findIndex(kid => kid.age < 5);

console.log(preschoolerIndex);
// 1

Parameters

1. callback
   Testing function to execute on each value in the array, taking 3 arguments:

   element
   The current element in the array.
   index (Optional)
   The index (position) of the current element in the array.
   array (Optional)
   The array that find was called on.

2. thisArg (Optional)
   Object to use as this inside callback.

The findIndex function is intentionally generic; it does not require that its this value be an Array object. Therefore it can be transferred to other kinds of objects for use as a method.