LESSON 118 OF 671 · ARRAYS - CHALLENGES

First n Items in JavaScript: Copying with slice

You get an array of integers nums and a number n. Return an array with the first n numbers of nums.

Keep them in the same order. When n is 0, return an empty array.

Example 1:

Input: nums = [3, 8, 1, 9, 4], n = 3
Output: [3, 8, 1]
Explanation: The first 3 numbers are 3, 8 and 1.
The 9 and the 4 are left out.

Example 2:

Input: nums = [5, 6, 7], n = 0
Output: []
Explanation: No numbers are taken, so the result is empty.

Example 3:

Input: nums = [2, 4], n = 2
Output: [2, 4]
Explanation: n is the length of nums, so every number is kept.

Constraints:

nums has 1 to 10 numbers.
Every number is an integer from -1000 to 1000.
n is from 0 to the length of nums.

STEP 1 OF 2

Guided

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JAVASCRIPT · ARRAYS - CHALLENGES

First N Items in JavaScript, explained

Problem overview

The method firstN receives an array of integers nums and a number n. It returns a new array with the first n numbers of nums, in the same order. When n is 0 the result is an empty array. The constraints say nums has 1 to 10 numbers and n is between 0 and the length of nums, so n never asks for more numbers than exist.

Three examples show the range of cases:

nums = [3, 8, 1, 9, 4] and n = 3 gives [3, 8, 1]. The 9 and the 4 are left out.
nums = [5, 6, 7] and n = 0 gives []. No numbers are taken.
nums = [2, 4] and n = 2 gives [2, 4]. Here n equals the length, so every number is kept.

Approach

You could build the result by hand: make an empty array, loop n times and push nums[i] each time. That works, but JavaScript already has a method for copying a part of an array. slice(start, end) copies the items from index start up to, but not including, index end, into a new array.

Because the end is left out, nums.slice(0, n) copies the items at indexes 0 to n - 1. That is exactly n items, the first n. The start is 0 because we begin at the front.

Step by step

Copy nums from index 0 up to index n with slice, and store the copy in a variable named result, declared with let.
Return result.

The edge cases need no extra code. With n = 0 the start and end are both 0, so the range is empty and slice returns []. With n equal to the length, the copy runs through the last item and returns every number.

The solution in JavaScript

class Solution {
    firstN(nums, n) {
        let result = nums.slice(0, n);
        return result;
    }
}

Details worth knowing

The result is a new array, so the caller's nums stays as it was. Pushing a new number onto the result does not touch the original: with nums = [3, 8, 1, 9, 4], a result of [3, 8, 1] that gets a 0 pushed becomes [3, 8, 1, 0], while nums is still [3, 8, 1, 9, 4].

The method returns the array itself, not text, and numbers stay numbers. If you print the arrays in your own file with console.log, they show as [ 3, 8, 1 ]. The lesson's tests compare the values.

Testing

Running the method on the three examples gives these results:

[ 3, 8, 1 ]
[]
[ 2, 4 ]

Also try n equal to 1, which should give an array with only the first number, and an array with one number and n = 1, which should give that same single number back.

Edge cases and common mistakes

Off by one with the end index. The end of slice is not included. nums.slice(0, n - 1) returns one number too few, and nums.slice(0, n + 1) returns one too many when there are enough numbers.
Using splice. nums.splice(0, n) also returns the first n numbers, but it removes them from nums. Use slice when the original must stay.
Forgetting to return. A method without a return gives back undefined, so the tests would see no array.
Special-casing zero. An if (n === 0) branch is not needed. The empty range already gives an empty array.

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