Simple function of Filter, Index and Every

Filter function

filter() method creates a new array with all elements that pass the test implemented by the provided function.

const elements = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

Let’s create isEven function.

function isEven(element) {
  return element % 2 === 0;
}

Use filter to find even numbers, reach documentation 🚀 filter.

elements.filter(isEven);

As complete codes.

function isEven(element) {
  return element % 2 === 0;
}

const elements = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

console.log(elements.filter(isEven));
// output [2, 4, 6, 8, 10]

Index function

indexOf() method returns the first index at which a given element can be found in the array, or -1 if it is not present.

const cities = ["jakarta", "texas", "tokyo"];

const index = cities.indexOf("tokyo");

//index is `2`

You may use findIndex for an Objects array.

const cities = [{ city: "jakarta" }, { city: "texas" }, { city: "tokyo" }];

const index = cities.findIndex((el, index) => {
  if (el.city === "texas") return true;
});

console.log(index); // index is 1

Every function

every() method tests whether all elements in the array pass the test implemented by the provided function.

[1, 2, 3, 4, 5].every(el => el < 10); // true
[1, 2, 3, 4, 5].every(el => el < 5); // false
[1, 2, 3, 4, 5].every(el => el > 0); // true