all: simplify some error variables

There's no need to put the error variables in a larger scope, nor define
them earlier than necessary. If anything, it makes the code harder to
follow, such as figuring out when nil errors are returned.

actions.go

@@ -28,14 +28,11 @@ type Tasks []Action
 // Do executes the list of Actions sequentially, using the provided context and
 // frame handler.
 func (t Tasks) Do(ctxt context.Context, h cdp.Handler) error {
-	var err error
-
 	// TODO: put individual task timeouts from context here
 	for _, a := range t {
 		// ctxt, cancel = context.WithTimeout(ctxt, timeout)
 		// defer cancel()
-		err = a.Do(ctxt, h)
+		if err := a.Do(ctxt, h); err != nil {
-		if err != nil {
 			return err
 		}
 	}

chromedp.go

@@ -42,8 +42,6 @@ type CDP struct {
 
 // New creates and starts a new CDP instance.
 func New(ctxt context.Context, opts ...Option) (*CDP, error) {
-	var err error
-
 	c := &CDP{
 		handlers:   make([]*TargetHandler, 0),
 		handlerMap: make(map[string]int),
@@ -54,14 +52,14 @@ func New(ctxt context.Context, opts ...Option) (*CDP, error) {
 
 	// apply options
 	for _, o := range opts {
-		err = o(c)
+		if err := o(c); err != nil {
-		if err != nil {
 			return nil, err
 		}
 	}
 
 	// check for supplied runner, if none then create one
 	if c.r == nil && c.watch == nil {
+		var err error
 		c.r, err = runner.Run(ctxt, c.opts...)
 		if err != nil {
 			return nil, err
@@ -121,8 +119,7 @@ func (c *CDP) AddTarget(ctxt context.Context, t client.Target) {
 	}
 
 	// run
-	err = h.Run(ctxt)
+	if err := h.Run(ctxt); err != nil {
-	if err != nil {
 		c.errorf("could not start handler for %s: %v", t, err)
 		return
 	}

eval.go

@@ -28,8 +28,6 @@ func Evaluate(expression string, res interface{}, opts ...EvaluateOption) Action
 	}
 
 	return ActionFunc(func(ctxt context.Context, h cdp.Handler) error {
-		var err error
-
 		// set up parameters
 		p := rundom.Evaluate(expression)
 		switch res.(type) {

handler.go

@@ -80,8 +80,6 @@ func NewTargetHandler(t client.Target, logf, debugf, errorf LogFunc) (*TargetHan
 //
 // Callers can stop Run by closing the passed context.
 func (h *TargetHandler) Run(ctxt context.Context) error {
-	var err error
-
 	// reset
 	h.Lock()
 	h.frames = make(map[cdp.FrameID]*cdp.Frame)
@@ -107,8 +105,7 @@ func (h *TargetHandler) Run(ctxt context.Context) error {
 		dom.Enable(),
 		css.Enable(),
 	} {
-		err = a.Do(ctxt, h)
+		if err := a.Do(ctxt, h); err != nil {
-		if err != nil {
 			return fmt.Errorf("unable to execute %s: %v", reflect.TypeOf(a), err)
 		}
 	}
@@ -175,25 +172,23 @@ func (h *TargetHandler) run(ctxt context.Context) {
 		}
 	}()
 
-	var err error
-
 	// process queues
 	for {
 		select {
 		case ev := <-h.qevents:
-			err = h.processEvent(ctxt, ev)
+			err := h.processEvent(ctxt, ev)
 			if err != nil {
 				h.errorf("could not process event %s: %v", ev.Method, err)
 			}
 
 		case res := <-h.qres:
-			err = h.processResult(res)
+			err := h.processResult(res)
 			if err != nil {
 				h.errorf("could not process result for message %d: %v", res.ID, err)
 			}
 
 		case cmd := <-h.qcmd:
-			err = h.processCommand(cmd)
+			err := h.processCommand(cmd)
 			if err != nil {
 				h.errorf("could not process command message %d: %v", cmd.ID, err)
 			}

pool.go

@@ -28,8 +28,6 @@ type Pool struct {
 
 // NewPool creates a new Chrome runner pool.
 func NewPool(opts ...PoolOption) (*Pool, error) {
-	var err error
-
 	p := &Pool{
 		start:  DefaultPoolStartPort,
 		end:    DefaultPoolEndPort,
@@ -41,13 +39,12 @@ func NewPool(opts ...PoolOption) (*Pool, error) {
 
 	// apply opts
 	for _, o := range opts {
-		err = o(p)
+		if err := o(p); err != nil {
-		if err != nil {
 			return nil, err
 		}
 	}
 
-	return p, err
+	return p, nil
 }
 
 // Shutdown releases all the pool resources.

query.go

@@ -445,8 +445,6 @@ func Screenshot(sel interface{}, picbuf *[]byte, opts ...QueryOption) Action {
 			return fmt.Errorf("selector `%s` did not return any nodes", sel)
 		}
 
-		var err error
-
 		// get box model
 		box, err := dom.GetBoxModel().WithNodeID(nodes[0].NodeID).Do(ctxt, h)
 		if err != nil {
@@ -596,9 +594,8 @@ func ScrollIntoView(sel interface{}, opts ...QueryOption) Action {
 			return fmt.Errorf("selector `%s` did not return any nodes", sel)
 		}
 
-		var err error
 		var pos []int
-		err = EvaluateAsDevTools(fmt.Sprintf(scrollIntoViewJS, nodes[0].FullXPath()), &pos).Do(ctxt, h)
+		err := EvaluateAsDevTools(fmt.Sprintf(scrollIntoViewJS, nodes[0].FullXPath()), &pos).Do(ctxt, h)
 		if err != nil {
 			return err
 		}

sel.go

@@ -221,9 +221,8 @@ func ByNodeID(s *Selector) {
 	}
 
 	ByFunc(func(ctxt context.Context, h cdp.Handler, n *cdp.Node) ([]cdp.NodeID, error) {
-		var err error
 		for _, id := range ids {
-			err = dom.RequestChildNodes(id).WithPierce(true).Do(ctxt, h)
+			err := dom.RequestChildNodes(id).WithPierce(true).Do(ctxt, h)
 			if err != nil {
 				return nil, err
 			}
@@ -296,10 +295,8 @@ func NodeReady(s *Selector) {
 // NodeVisible is a query option to wait until the element is visible.
 func NodeVisible(s *Selector) {
 	WaitFunc(s.waitReady(func(ctxt context.Context, h cdp.Handler, n *cdp.Node) error {
-		var err error
-
 		// check box model
-		_, err = dom.GetBoxModel().WithNodeID(n.NodeID).Do(ctxt, h)
+		_, err := dom.GetBoxModel().WithNodeID(n.NodeID).Do(ctxt, h)
 		if err != nil {
 			if isCouldNotComputeBoxModelError(err) {
 				return ErrNotVisible
@@ -324,10 +321,8 @@ func NodeVisible(s *Selector) {
 // NodeNotVisible is a query option to wait until the element is not visible.
 func NodeNotVisible(s *Selector) {
 	WaitFunc(s.waitReady(func(ctxt context.Context, h cdp.Handler, n *cdp.Node) error {
-		var err error
-
 		// check box model
-		_, err = dom.GetBoxModel().WithNodeID(n.NodeID).Do(ctxt, h)
+		_, err := dom.GetBoxModel().WithNodeID(n.NodeID).Do(ctxt, h)
 		if err != nil {
 			if isCouldNotComputeBoxModelError(err) {
 				return nil