One array is given and a number is given. Find the nearest value of that number in the array in one line. Try not to use a for loop.
Find the nearest element in an array in JavaScript with one line
const findNearest = (arr, num) =>
givenArray.reduce(
(acc, value) => (Math.abs(acc  num) < Math.abs(value  num) ? acc : value),
arr[0]
);
const givenArray = [1, 4, 6, 8, 9, 15, 21];
console.log(findNearest(givenArray, 2));
console.log(findNearest(givenArray, 11));
console.log(findNearest(givenArray, 14));
console.log(findNearest(givenArray, 16));
console.log(findNearest(givenArray, 20));
Explanation:

findNearest takes an array and returns the nearest value to num in that array.

We are using reduce. It takes one function as the first param and calculate one single value from the values of the array

acc is the accumulator i.e. it computes and holds the final value and value is the current value in the array.

We are using Math.abs to find the absolute value.

On each step, it tries to find the closest element to num by comparing the difference between accumulator and current value with current value and given number.
It prints :
1
9
15
15
21