Array.prototype.filter()

callbackFn

A function to execute for each element in the array. It should return a truthy value to keep the element in the resulting array, and a falsy value otherwise. The function is called with the following arguments:

element

The current element being processed in the array.

index

The index of the current element being processed in the array.

array

The array filter() was called upon.

thisArg Optional

A value to use as this when executing callbackFn. See iterative methods.

A shallow copy of the given array containing just the elements that pass the test. If no elements pass the test, an empty array is returned.

The filter() method is an iterative method. It calls a provided callbackFn function once for each element in an array, and constructs a new array of all the values for which callbackFn returns a truthy value. Array elements which do not pass the callbackFn test are not included in the new array. Read the iterative methods section for more information about how these methods work in general.

callbackFn is invoked only for array indexes which have assigned values. It is not invoked for empty slots in sparse arrays.

The filter() method is generic. It only expects the this value to have a length property and integer-keyed properties.

The following example uses filter() to create a filtered array that has all elements with values less than 10 removed.

function isBigEnough(value) {
  return value >= 10;
}

const filtered = [12, 5, 8, 130, 44].filter(isBigEnough);
// filtered is [12, 130, 44]

The following example returns all prime numbers in the array:

const array = [-3, -2, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13];

function isPrime(num) {
  for (let i = 2; num > i; i++) {
    if (num % i === 0) {
      return false;
    }
  }
  return num > 1;
}

console.log(array.filter(isPrime)); // [2, 3, 5, 7, 11, 13]

The following example uses filter() to create a filtered JSON of all elements with non-zero, numeric id.

const arr = [
  { id: 15 },
  { id: -1 },
  { id: 0 },
  { id: 3 },
  { id: 12.2 },
  {},
  { id: null },
  { id: NaN },
  { id: "undefined" },
];

let invalidEntries = 0;

function filterByID(item) {
  if (Number.isFinite(item.id) && item.id !== 0) {
    return true;
  }
  invalidEntries++;
  return false;
}

const arrByID = arr.filter(filterByID);

console.log("Filtered Array\n", arrByID);
// Filtered Array
// [{ id: 15 }, { id: -1 }, { id: 3 }, { id: 12.2 }]

console.log("Number of Invalid Entries =", invalidEntries);
// Number of Invalid Entries = 5

Following example uses filter() to filter array content based on search criteria.

const fruits = ["apple", "banana", "grapes", "mango", "orange"];

/**
 * Filter array items based on search criteria (query)
 */
function filterItems(arr, query) {
  return arr.filter((el) => el.toLowerCase().includes(query.toLowerCase()));
}

console.log(filterItems(fruits, "ap")); // ['apple', 'grapes']
console.log(filterItems(fruits, "an")); // ['banana', 'mango', 'orange']

The array argument is useful if you want to access another element in the array, especially when you don't have an existing variable that refers to the array. The following example first uses map() to extract the numerical ID from each name and then uses filter() to select the ones that are greater than its neighbors.

const names = ["JC63", "Bob132", "Ursula89", "Ben96"];
const greatIDs = names
  .map((name) => parseInt(name.match(/[0-9]+/)[0], 10))
  .filter((id, idx, arr) => {
    // Without the arr argument, there's no way to easily access the
    // intermediate array without saving it to a variable.
    if (idx > 0 && id <= arr[idx - 1]) return false;
    if (idx < arr.length - 1 && id <= arr[idx + 1]) return false;
    return true;
  });
console.log(greatIDs); // [132, 96]

The array argument is not the array that is being built — there is no way to access the array being built from the callback function.

filter() will skip empty slots.

console.log([1, , undefined].filter((x) => x === undefined)); // [undefined]
console.log([1, , undefined].filter((x) => x !== 2)); // [1, undefined]

The filter() method reads the length property of this and then accesses each property whose key is a nonnegative integer less than length.

const arrayLike = {
  length: 3,
  0: "a",
  1: "b",
  2: "c",
  3: "a", // ignored by filter() since length is 3
};
console.log(Array.prototype.filter.call(arrayLike, (x) => x <= "b"));
// [ 'a', 'b' ]

• Polyfill of Array.prototype.filter in core-js
• Indexed collections guide
• Array
• Array.prototype.forEach()
• Array.prototype.every()
• Array.prototype.map()
• Array.prototype.some()
• Array.prototype.reduce()
• TypedArray.prototype.filter()