Difference Between indexOf and findIndex function of array

Simple - What kind of array structure are you using?

• If array of objects, findIndex();
• Else, indexOf().

"I want to find the index in an array of objects, with the key of "Orange".

let fruits = [
   { type: "Apple", quantity: 9 },
   { type: "Banana", quantity: 2},
   { type: "Orange", quantity: 8},
   { type: "Pear", quantity: 777}
];

let myIndex = fruits.findIndex(fruit => fruit.type === "Orange"); // Returns 2.

"I want to find the index in a simple array".

let fruits = [ "Apple", "Banana", "Pear", "Orange"];

let index = fruits.indexOf("Orange"); // Returns 3.

FindIndex is useful if you want to find the first element that matches to your predicate: In W3C's example, there are numbers and matches if the customer's age above or equals to 18.

var ages = [3, 10, 18, 20];

function checkAdult(age) {
    return age >= 18;
}

console.log(ages.findIndex(checkAdult));

console:

2

You can find an exact element index with the indexOf function of Array, but you can't pass a predicate. It is faster if you want to find a specific element:

var ages = [3, 10, 18, 20];
console.log(ages.indexOf(10));

returns:

1

Index counting starts at 0, so the first element index is 0.

The main difference are the parameters of these functions:

• Array.prototype.indexOf() expects a value as first parameter. This makes it a good choice to find the index in arrays of primitive types (like string, number, or boolean).

• Array.prototype.findIndex() expects a callback as first parameter. Use this if you need the index in arrays with non-primitive types (e.g. objects) or your find condition is more complex than just a value.

See the links for examples of both cases.