How to remove all element from array except the first one in javascript

This is the head function. tail is also demonstrated as a complimentary function.

Note, you should only use head and tail on arrays that have a known length of 1 or more.

// head :: [a] -> a
const head = ([x,...xs]) => x;

// tail :: [a] -> [a]
const tail = ([x,...xs]) => xs;

let input = ['a','b','c','d','e','f'];

console.log(head(input)); // => 'a'
console.log(tail(input)); // => ['b','c','d','e','f']

array = [a,b,c,d,e,f];
remaining = array[0];
array = [remaining];

You can set the length property of the array.

var input = ['a','b','c','d','e','f'];  
input.length = 1;
console.log(input);

OR, Use splice(startIndex) method

var input = ['a','b','c','d','e','f'];  
input.splice(1);
console.log(input);

OR use Array.slice method

var input = ['a','b','c','d','e','f'];  
var output = input.slice(0, 1) // 0-startIndex, 1 - endIndex
console.log(output);