What does "=>" mean in JavaScript? [duplicate]

Question

Here is the code:

function accum(s) {
  return s.split('').map((x,index) => x.toUpperCase()+Array(index+1).join(x.toLowerCase())).join('-');
}

I would like to know what "=>" is. This function takes a string and for the index number of each element it adds that many elements to the output. Here's an example:

accum("abcd") --> "A-Bb-Ccc-Dddd"
accum("RqaEzty") --> "R-Qq-Aaa-Eeee-Zzzzz-Tttttt-Yyyyyyy"
accum("cwAt") --> "C-Ww-Aaa-Tttt"

Answer 1

It's a new feature that introduced in ES6 and is called arrow function. The left part denotes the input of a function and the right part the output of that function.

So in your case

s.split('')

splits the input on empty spaces and for each element of the resulted array you apply the following function:

(x,index) => x.toUpperCase()+Array(index+1).join(x.toLowerCase())

The left part is the random element, x of the array (s.split('')) and it's corresponding index. The second part applies a transformation to this input.