Photoshop grid selection

select-grid.js

// Get a reference to the active document.
var doc = app.activeDocument;

// Set the height of the selection to the height of the document.
var height = doc.height;

// Set the x and y offset in pixels.
var xOffset = 5;
var yOffset = 5;

// Remove any existing selections.
doc.selection.deselect();

// Set the size of the grid.
var gridSize = 100;

// Set the horizontal and vertical margins between the grids.
var horizontalMargin = 100;
var verticalMargin = 100;

// Loop through the document width or height with a step of gridSize + horizontalMargin.
for(var x = 0; x < doc.width; x += gridSize + horizontalMargin) {
  for(var y = 0; y < doc.height; y += gridSize + verticalMargin) {
    // Create a selection that is gridSize wide and gridSize high.
    doc.selection.select([
      [x + xOffset, y + yOffset],                         // Top left corner
      [x + xOffset + gridSize, y + yOffset],              // Top right corner
      [x + xOffset + gridSize, y + gridSize + yOffset],   // Bottom right corner
      [x + xOffset, y + gridSize + yOffset]               // Bottom left corner
    ], SelectionType.EXTEND);
  }
}