Thank you! This was super useful :)

Hey there. I like your code very much. It made my day. Thank you very very much. But I think there is a problem. If I mask any layer and then try to run this script, it is not working properly. Will you please take a look?

Thank you jawinn! Really useful script. But it doesn't fit layer proportionally. I fixed it:

// FIT LAYER TO CANVAS
// via https://forums.adobe.com/message/5413957#5413957
var maintainAspectRatio = true; // set to true to keep aspect ratio

if (app.documents.length > 0) {
app.activeDocument.suspendHistory('Fit Layer to Canvas', 'FitLayerToCanvas(' + maintainAspectRatio + ')');
}

function FitLayerToCanvas(keepAspect) {
// keepAspect: Boolean - optional. Default to false
var doc = app.activeDocument;
var layer = doc.activeLayer;

// do nothing if layer is background or locked
if (layer.isBackgroundLayer || layer.allLocked || layer.pixelsLocked || layer.positionLocked || layer.transparentPixelsLocked) return;

// do nothing if layer is not a normal artLayer or Smart Object
if (layer.kind != LayerKind.NORMAL && layer.kind != LayerKind.SMARTOBJECT) return;

// store the ruler
var defaultRulerUnits = app.preferences.rulerUnits;
app.preferences.rulerUnits = Units.PIXELS;

var width = doc.width.as('px');
var height = doc.height.as('px');
var bounds = app.activeDocument.activeLayer.bounds;
var layerWidth = bounds[2].as('px') - bounds[0].as('px');
var layerHeight = bounds[3].as('px') - bounds[1].as('px');

// move the layer so the top-left corner matches the canvas top-left corner
layer.translate(new UnitValue(0 - bounds[0].as('px'), 'px'), new UnitValue(0 - bounds[1].as('px'), 'px'));

if (keepAspect) {
var layerRatio = layerWidth / layerHeight;
var canvasRatio = width / height;

if (layerRatio > canvasRatio) {
  var newWidth = width;
  var newHeight = newWidth / layerRatio;
} else {
  var newHeight = height;
  var newWidth = newHeight * layerRatio;
}

var resizePercent = (newWidth / layerWidth) * 100;
app.activeDocument.activeLayer.resize(resizePercent, resizePercent, AnchorPosition.TOPLEFT);

} else {
// scale the layer to match the canvas
layer.resize((width / layerWidth) * 100, (height / layerHeight) * 100, AnchorPosition.TOPLEFT);
}

// restore the ruler
app.preferences.rulerUnits = defaultRulerUnits;
}