Javascript数组从数组中的每个元素中减去一个值

Question

I have the following code written and I have no idea why it is not working. Essentially, I am trying to subtract 255 from every value in an array, but it must be ordered where 255 is first (I can't just do i - 255, it won't get me the values I need). I have also tried i+-255 to no avail.

function makeReverse(original, output){
  for (i=0; i<original.length; i++) {
    output[i]=original[i]; //copy original array to output array
    if ((i % 4) !== 3) {
      output[i] = output[255 - i]; //why this is not working is beyond me the 
                                     values keep reading 0 for rgb
    }
  }
  console.log(output);
  return output;
}

Answer 1

Easiest way would be using map . Something like this:

original.map(function(element){
   return element - 255;
});

Or in ES6:

original.map(element => element - 255);

Or if you only want to subtract from the value under a condition:

original.map(function(value){
   if(value % 4 !== 3){
       return value - 255;
   } else {
       return value;
   }
});

Check here to find out how map works.

Answer 2

why you subtracting from index [i-255] . subtract it from value like this

output[i] = output[i]-255; 

Answer 3

I think you are looking for

output[i] = original[255 - i];

This would make your output array the reverse-order of the original array (for elements whose index is not 3 mod 4).