(refactor): use const everywhere instead of let

- none of these variables are ever re-assigned
  - I wrote this a while ago when ES6 was new and just used `let` for
    everything instead of `var` at the time. Now I use `const` instead
    😂... and `let` where needed

Author: agilgur5

index.es6

@@ -1,24 +1,24 @@
 export default function trimCanvas (canvas) {
-  let context = canvas.getContext('2d')
+  const context = canvas.getContext('2d')
 
-  let imgWidth = canvas.width
-  let imgHeight = canvas.height
+  const imgWidth = canvas.width
+  const imgHeight = canvas.height
 
-  let imgData = context.getImageData(0, 0, imgWidth, imgHeight).data
+  const imgData = context.getImageData(0, 0, imgWidth, imgHeight).data
 
   // get the corners of the relevant content (everything that's not white)
-  let cropTop = scanY(true, imgWidth, imgHeight, imgData)
-  let cropBottom = scanY(false, imgWidth, imgHeight, imgData)
-  let cropLeft = scanX(true, imgWidth, imgHeight, imgData)
-  let cropRight = scanX(false, imgWidth, imgHeight, imgData)
+  const cropTop = scanY(true, imgWidth, imgHeight, imgData)
+  const cropBottom = scanY(false, imgWidth, imgHeight, imgData)
+  const cropLeft = scanX(true, imgWidth, imgHeight, imgData)
+  const cropRight = scanX(false, imgWidth, imgHeight, imgData)
 
   // + 1 is needed because this is a difference, there are n + 1 pixels in
   // between the two numbers inclusive
-  let cropXDiff = (cropRight - cropLeft) + 1
-  let cropYDiff = (cropBottom - cropTop) + 1
+  const cropXDiff = (cropRight - cropLeft) + 1
+  const cropYDiff = (cropBottom - cropTop) + 1
 
   // get the relevant data from the calculated coordinates
-  let trimmedData = context.getImageData(cropLeft, cropTop, cropXDiff,
+  const trimmedData = context.getImageData(cropLeft, cropTop, cropXDiff,
     cropYDiff)
 
   // set the trimmed width and height
@@ -48,8 +48,8 @@ function getAlpha (x, y, imgWidth, imgData) {
 
 // finds the first y coord in imgData that is not white
 function scanY (fromTop, imgWidth, imgHeight, imgData) {
-  let offset = fromTop ? 1 : -1
-  let firstCol = fromTop ? 0 : imgHeight - 1
+  const offset = fromTop ? 1 : -1
+  const firstCol = fromTop ? 0 : imgHeight - 1
 
   // loop through each row
   for (var y = firstCol; fromTop ? (y < imgHeight) : (y > -1); y += offset) {
@@ -68,8 +68,8 @@ function scanY (fromTop, imgWidth, imgHeight, imgData) {
 
 // finds the first x coord in imgData that is not white
 function scanX (fromLeft, imgWidth, imgHeight, imgData) {
-  let offset = fromLeft ? 1 : -1
-  let firstRow = fromLeft ? 0 : imgWidth - 1
+  const offset = fromLeft ? 1 : -1
+  const firstRow = fromLeft ? 0 : imgWidth - 1
 
   // loop through each column
   for (var x = firstRow; fromLeft ? (x < imgWidth) : (x > -1); x += offset) {