Get the index of an Object in an Array in JavaScript

Get the index of an Object in an Array in JavaScript

To find the index of an object in an array by a specific property:

1. Use the findIndex() method to iterate over the array.
2. Check if each object has a property with the specified value.
3. The findIndex() method will return the index of the first object that matches the condition.

const arr = [{id: 'a'}, {id: 'b'}, {id: 'c'}];

const index = arr.findIndex(object => {
  return object.id === 'b';
});

console.log(index); // 👉️ 1

The function we passed to the Array.findIndex() method gets called with each element (object) in the array until it returns a truthy value or iterates over all array elements.

On each iteration, we check if the object’s property is equal to a specific value and return true or false.

The findIndex() method returns the index of the first object that meets the condition.

If the function we passed to the findIndex() method never returns a truthy value, the method returns -1.

Alternatively, you can use the Array.map() method.

Get the index of an Object in an Array using Array.map()

This is a three-step process:

1. Use the map() method to iterate over the array.
2. Return only the values of the relevant property.
3. Use the indexOf() method to get the index of the object in the array.

const arr = [{id: 'a'}, {id: 'b'}, {id: 'c'}];

const index = arr.map(object => object.id).indexOf('c');

console.log(index); // 👉️ 2

We used the Array.map() method to get an array containing the values of the specified property.

const arr = [{id: 'a'}, {id: 'b'}, {id: 'c'}];

const values = arr.map(object => object.id)
console.log(values) // 👉️ ['a', 'b', 'c']

The last step is to use the Array.indexOf method to get the index of the value in the array.

The map() method iterates over all of the array’s elements, so the ordering of the elements is preserved.

The order of the elements is the same for both the array of values and the array of objects.

The indexOf method returns the index of the first object that meets the condition.

If the indexOf() method doesn’t find an element with the given value, it returns -1, just like the findIndex() method.

Get the Indexes of all Objects in an Array that match a condition

To get all indexes of all objects in an array that match a condition:

1. Use the map() method to iterate over the array.
2. Check if each object matches the condition and return the matching indexes.
3. Use the filter() method to remove all undefined values from the array.

const arr = [{name: 'Alice'}, {name: 'Bob'}, {name: 'Charlie'}];

const indexes = arr
  .map((element, index) => {
    if (element.name === 'Alice' || element.name === 'Bob') {
      return index;
    }
  })
  .filter(element => element >= 0);

console.log(indexes); // 👉️ [0 , 1]

The function we passed to the Array.map() method gets called with each element (object) in the array.

If the condition is met, we return the element’s index, otherwise, the function implicitly returns undefined.

The return value of the map() method will contain the indexes of the array elements that meet the condition and an undefined value for each element that doesn’t.

const arr = [{name: 'Alice'}, {name: 'Bob'}, {name: 'Charlie'}];

// 👇️ [0, 1, undefined]
console.log(
  arr.map((element, index) => {
    if (element.name === 'Alice' || element.name === 'Bob') {
      return index;
    }
  }),
);

We used the Array.filter method to remove the undefined values from the array.

In the callback function we passed to the filter() method, we check if the element is greater than or equal to 0.

The filter method returns a new array that contains the indexes of all elements that meet the condition.

You can also use the Array.some() method to get the index of an object in an array.

Get the index of an Object in an Array using Array.some()

This is a three-step process:

1. Use the Array.some() method to iterate over the array.
2. Check if the current object has a property equal to the specified value.
3. If the condition is met, assign the current index to a variable.

const arr = [{id: 'a'}, {id: 'b'}, {id: 'c'}];

let index;

arr.some((object, idx) => {
  if (object.id === 'b') {
    index = idx;
    return true;
  }
});

console.log(index); // 👉️ 1

The function we passed to the Array.some() method gets called with each element (object) in the array.

If at least one invocation of the callback function returns a truthy value, the some() method returns true, otherwise, false is returned.

On each iteration, we check if the current object has an id property with the specified value.

If the condition is met, we assign the current index to the index variable and return true to break out of the loop.

You can initialize the index variable to a different value, e.g. -1 if you want the variable to store -1 in case an object with the specified value isn’t found.

const arr = [{id: 'a'}, {id: 'b'}, {id: 'c'}];

let index = -1;

arr.some((object, idx) => {
  if (object.id === 'ASDF') {
    index = idx;
    return true;
  }
});

console.log(index); // 👉️ -1

None of the objects in the array meets the condition, so the index variable remains set to -1.

Get the index of an Object in an Array using a for loop

This is a three-step process:

1. Use a for loop to iterate over the array.
2. Check if the current object has a property with the specified value.
3. If the condition is met, assign the current index to a variable.

const arr = [{id: 'a'}, {id: 'b'}, {id: 'c'}];

let index;

for (let idx = 0; idx < arr.length; idx++) {
  if (arr[idx].id === 'b') {
    index = idx;
    break;
  }
}

console.log(index); // 👉️ 1

We used a for loop to iterate over the array of objects.

On each iteration, we check if the current object has an id property equal to a specific value.

If the condition is met, we assign the current index to a variable and exit the for loop.

The break statement is used to end the for loop prematurely to avoid iterating needlessly.