const numbers = [1, 2, 3, 4];
const filteredNumbers = numbers.map((num, index) => {
if (index < 3) {
return num;
}
});
// filteredNumbers is [1, 2, 3, undefined]
According to my understanding callback function should return all numbers of array if their index is less than 3 so it should return 1,2,3 and stop after that and number 4 can not be returned as if condition says index should be less than 3. I am wondering why it returned undefined for index 3 number.
.map is an isomorphism and preserves the length of the given array.
So if the callback function does not return anything, the index will be undefined.
You want to user a filter function instead if you need to filter array elements that meet a certain condition.
const numbers = [1, 2, 3, 4];
const filteredNumbers = numbers.filter((_, index) => index < 3);
console.log(filteredNumbers);If you want just to get the first 3 items in the array, I recommend using slice.
const numbers = [1, 2, 3, 4];
console.log(numbers.slice(0, 3));Second solution is use filter instead of map.
const numbers = [1, 2, 3, 4];
const filteredNumbers = numbers.filter((num, index) => {
return index < 3
});
console.log(filteredNumbers)