feat(docs): add template preview (#1777)

.github/workflows/publish-docs.yml

@@ -17,6 +17,12 @@ jobs:
         uses: actions/checkout@v4
         with:
           fetch-depth: 0
+      - name: Set up Go
+        uses: actions/setup-go@v4
+        with:
+          go-version: 1.18.x
+      - name: Build tplprev
+        run: scripts/build-tplprev.sh
       - name: Setup python
         uses: actions/setup-python@v4
         with:

.gitignore

@@ -8,3 +8,6 @@ dist
 /site
 coverage.out
 *.coverprofile
+
+docs/assets/wasm_exec.js
+docs/assets/*.wasm

docs/template-preview.md

@@ -0,0 +1,219 @@
+<style>
+    #tplprev {
+        margin: 0;
+        display: flex; 
+        flex-direction: column; 
+        row-gap: 1rem; 
+        box-sizing: border-box; 
+        position: relative; 
+        margin-right: -13.3rem
+    }
+    #tplprev textarea {
+        box-decoration-break: slice;
+        overflow: auto;
+        padding: 0.77em 1.18em;
+        scrollbar-color: var(--md-default-fg-color--lighter) transparent;
+        scrollbar-width: thin;
+        touch-action: auto;
+        word-break: normal;
+        height: 420px;
+        flex: 1;
+    }
+    #tplprev .controls {
+        display: flex; 
+        flex-direction: row; 
+        column-gap: 0.5rem
+    }
+    #tplprev textarea, #tplprev input {
+        background-color: var(--md-code-bg-color);
+        border-width: 0;
+        border-radius: 0.1rem;
+        color: var(--md-code-fg-color);
+        font-feature-settings: "kern";
+        font-family: var(--md-code-font-family);
+    }
+    .numfield {
+        font-size: .7rem;
+        display: flex;
+        flex-direction: column;
+        justify-content: space-between;
+    }
+    #tplprev button {
+        border-radius: 0.1rem;
+        color: var(--md-typeset-color);
+        background-color: var(--md-primary-fg-color);
+        flex:1; 
+        min-width: 12ch; 
+        padding: 0.5rem
+    }
+    #tplprev button:hover {
+        background-color: var(--md-accent-fg-color);
+    }
+    #tplprev input[type="number"] { width: 5ch; flex: 1; font-size: 1rem; }
+    #tplprev fieldset {
+        margin-top: -0.5rem;
+        display: flex;
+        flex: 1;
+        column-gap: 0.5rem;
+    }
+    #tplprev .template-wrapper {
+        display: flex; 
+        flex:1; 
+        column-gap: 1rem;
+    }
+    #tplprev .result-wrapper {
+        flex: 1; 
+        display: flex
+    }
+    #result {
+        font-size: 0.7rem;
+        background-color: var(--md-code-bg-color);
+        scrollbar-color: var(--md-default-fg-color--lighter) transparent;
+        scrollbar-width: thin;
+        touch-action: auto;
+        overflow: auto;
+        padding: 0.77em 1.18em;
+        margin:0;
+        height: 540px;
+        flex:1; 
+        width:100%
+    }
+    #tplprev .loading {
+        position: absolute; 
+        inset: 0; 
+        display: flex; 
+        padding: 1rem; 
+        box-sizing: border-box; 
+        background: var(--md-code-bg-color); 
+        margin-top: 0
+    }
+</style>
+<script src="../assets/wasm_exec.js"></script>
+<script>
+    const updatePreview = () => {
+        const form = document.querySelector('#tplprev');
+        const input = form.template.value;
+        console.log('Input: %o', input);
+        const arrFromCount = (key) => Array.from(Array(form[key]?.valueAsNumber ?? 0), () => key);
+        const states = form.enablereport.checked ? [
+            ...arrFromCount("skipped"),
+            ...arrFromCount("scanned"),
+            ...arrFromCount("updated"),
+            ...arrFromCount("failed" ),
+            ...arrFromCount("fresh"  ),
+            ...arrFromCount("stale"  ),
+        ] : [];
+        console.log("States: %o", states);
+        const levels = form.enablelog.checked ? [
+            ...arrFromCount("error"),
+            ...arrFromCount("warning"),
+            ...arrFromCount("info"),
+            ...arrFromCount("debug"),
+        ] : [];
+        console.log("Levels: %o", levels);
+        const output = WATCHTOWER.tplprev(input, states, levels);
+        console.log('Output: \n%o', output);
+        if (output.length) {
+            document.querySelector('#result').innerText = output;
+        } else {
+            document.querySelector('#result').innerHTML = '<i>empty (would not be sent as a notification)</i>';
+        }
+    }
+    const formSubmitted = (e) => {
+        e.preventDefault();
+        updatePreview();
+    }
+    let debounce;
+    const inputUpdated = () => {
+        if(debounce) clearTimeout(debounce);
+        debounce = setTimeout(() => updatePreview(), 400);
+    }
+    const formChanged = (e) =>  {
+        console.log('form changed: %o', e);
+    }
+    const go = new Go();
+    WebAssembly.instantiateStreaming(fetch("../assets/tplprev.wasm"), go.importObject).then((result) => {
+        document.querySelector('#tplprev .loading').style.display = "none";
+        go.run(result.instance);
+        updatePreview();
+    });
+</script>
+<form id="tplprev" onchange="updatePreview()" onsubmit="formSubmitted(event)">
+<pre class="loading">loading wasm...</pre>
+<div class="template-wrapper">
+<textarea name="template" type="text" style="flex: 1" onkeyup="inputUpdated()">{{- with .Report -}}
+  {{- if ( or .Updated .Failed ) -}}
+{{len .Scanned}} Scanned, {{len .Updated}} Updated, {{len .Failed}} Failed
+    {{- range .Updated}}
+- {{.Name}} ({{.ImageName}}): {{.CurrentImageID.ShortID}} updated to {{.LatestImageID.ShortID}}
+    {{- end -}}
+    {{- range .Fresh}}
+- {{.Name}} ({{.ImageName}}): {{.State}}
+    {{- end -}}
+    {{- range .Skipped}}
+- {{.Name}} ({{.ImageName}}): {{.State}}: {{.Error}}
+    {{- end -}}
+    {{- range .Failed}}
+- {{.Name}} ({{.ImageName}}): {{.State}}: {{.Error}}
+      {{- end -}}
+  {{- end -}}
+{{- end -}}
+{{- if (and .Entries .Report) }}
+
+Logs:
+{{ end -}}
+{{range .Entries -}}{{.Time.Format "2006-01-02T15:04:05Z07:00"}} [{{.Level}}] {{.Message}}{{"\n"}}{{- end -}}</textarea>
+</div>
+<div class="controls">
+<fieldset>
+    <legend><label><input type="checkbox" name="enablereport" checked /> Container report</label></legend>
+    <label class="numfield">
+        Skipped:
+        <input type="number" name="skipped" value="3" />
+    </label>
+    <label class="numfield">
+        Scanned:
+        <input type="number" name="scanned" value="3" />
+    </label>
+    <label class="numfield">
+        Updated:
+        <input type="number" name="updated" value="3" />
+    </label>
+    <label class="numfield">
+        Failed:
+        <input type="number" name="failed" value="3" />
+    </label>
+    <label class="numfield">
+        Fresh:
+        <input type="number" name="fresh" value="3" />
+    </label>
+    <label class="numfield">
+        Stale:
+        <input type="number" name="stale" value="3" />
+    </label>
+</fieldset>
+<fieldset>
+    <legend><label><input type="checkbox" name="enablelog" checked /> Log entries</label></legend>
+    <label class="numfield">
+        Error: 
+        <input type="number" name="error" value="1" />
+    </label>
+    <label class="numfield">
+        Warning:
+        <input type="number" name="warning" value="2" />
+    </label>
+    <label class="numfield">
+        Info:
+        <input type="number" name="info" value="3" />
+    </label>
+    <label class="numfield">
+        Debug:
+        <input type="number" name="debug" value="4" />
+    </label>
+</fieldset>
+<button type="submit">Update preview</button>
+</div>
+<div style="result-wrapper">
+    <pre id="result"></pre>
+</div>
+</form>

pkg/notifications/json.go

@@ -59,13 +59,3 @@ func marshalReports(reports []t.ContainerReport) []jsonMap {
 }
 
 var _ json.Marshaler = &Data{}
-
-func toJSON(v interface{}) string {
-	var bytes []byte
-	var err error
-	if bytes, err = json.MarshalIndent(v, "", "  "); err != nil {
-		LocalLog.Errorf("failed to marshal JSON in notification template: %v", err)
-		return ""
-	}
-	return string(bytes)
-}

pkg/notifications/preview/data/data.go

@@ -0,0 +1,143 @@
+package data
+
+import (
+	"encoding/hex"
+	"errors"
+	"math/rand"
+	"strconv"
+	"time"
+
+	"github.com/containrrr/watchtower/pkg/types"
+)
+
+type previewData struct {
+	rand           *rand.Rand
+	lastTime       time.Time
+	report         *report
+	containerCount int
+	Entries        []*logEntry
+	StaticData     staticData
+}
+
+type staticData struct {
+	Title string
+	Host  string
+}
+
+// New initializes a new preview data struct
+func New() *previewData {
+	return &previewData{
+		rand:           rand.New(rand.NewSource(1)),
+		lastTime:       time.Now().Add(-30 * time.Minute),
+		report:         nil,
+		containerCount: 0,
+		Entries:        []*logEntry{},
+		StaticData: staticData{
+			Title: "Title",
+			Host:  "Host",
+		},
+	}
+}
+
+// AddFromState adds a container status entry to the report with the given state
+func (pb *previewData) AddFromState(state State) {
+	cid := types.ContainerID(pb.generateID())
+	old := types.ImageID(pb.generateID())
+	new := types.ImageID(pb.generateID())
+	name := pb.generateName()
+	image := pb.generateImageName(name)
+	var err error
+	if state == FailedState {
+		err = errors.New(pb.randomEntry(errorMessages))
+	} else if state == SkippedState {
+		err = errors.New(pb.randomEntry(skippedMessages))
+	}
+	pb.addContainer(containerStatus{
+		containerID:   cid,
+		oldImage:      old,
+		newImage:      new,
+		containerName: name,
+		imageName:     image,
+		error:         err,
+		state:         state,
+	})
+}
+
+func (pb *previewData) addContainer(c containerStatus) {
+	if pb.report == nil {
+		pb.report = &report{}
+	}
+	switch c.state {
+	case ScannedState:
+		pb.report.scanned = append(pb.report.scanned, &c)
+	case UpdatedState:
+		pb.report.updated = append(pb.report.updated, &c)
+	case FailedState:
+		pb.report.failed = append(pb.report.failed, &c)
+	case SkippedState:
+		pb.report.skipped = append(pb.report.skipped, &c)
+	case StaleState:
+		pb.report.stale = append(pb.report.stale, &c)
+	case FreshState:
+		pb.report.fresh = append(pb.report.fresh, &c)
+	default:
+		return
+	}
+	pb.containerCount += 1
+}
+
+// AddLogEntry adds a preview log entry of the given level
+func (pd *previewData) AddLogEntry(level LogLevel) {
+	var msg string
+	switch level {
+	case FatalLevel:
+		fallthrough
+	case ErrorLevel:
+		fallthrough
+	case WarnLevel:
+		msg = pd.randomEntry(logErrors)
+	default:
+		msg = pd.randomEntry(logMessages)
+	}
+	pd.Entries = append(pd.Entries, &logEntry{
+		Message: msg,
+		Data:    map[string]any{},
+		Time:    pd.generateTime(),
+		Level:   level,
+	})
+}
+
+// Report returns a preview report
+func (pb *previewData) Report() types.Report {
+	return pb.report
+}
+
+func (pb *previewData) generateID() string {
+	buf := make([]byte, 32)
+	_, _ = pb.rand.Read(buf)
+	return hex.EncodeToString(buf)
+}
+
+func (pb *previewData) generateTime() time.Time {
+	pb.lastTime = pb.lastTime.Add(time.Duration(pb.rand.Intn(30)) * time.Second)
+	return pb.lastTime
+}
+
+func (pb *previewData) randomEntry(arr []string) string {
+	return arr[pb.rand.Intn(len(arr))]
+}
+
+func (pb *previewData) generateName() string {
+	index := pb.containerCount
+	if index <= len(containerNames) {
+		return "/" + containerNames[index]
+	}
+	suffix := index / len(containerNames)
+	index %= len(containerNames)
+	return "/" + containerNames[index] + strconv.FormatInt(int64(suffix), 10)
+}
+
+func (pb *previewData) generateImageName(name string) string {
+	index := pb.containerCount % len(organizationNames)
+	return organizationNames[index] + name + ":latest"
+}

pkg/notifications/preview/data/logs.go

@@ -0,0 +1,56 @@
+package data
+
+import (
+	"time"
+)
+
+type logEntry struct {
+	Message string
+	Data    map[string]any
+	Time    time.Time
+	Level   LogLevel
+}
+
+// LogLevel is the analog of logrus.Level
+type LogLevel string
+
+const (
+	TraceLevel LogLevel = "trace"
+	DebugLevel LogLevel = "debug"
+	InfoLevel  LogLevel = "info"
+	WarnLevel  LogLevel = "warning"
+	ErrorLevel LogLevel = "error"
+	FatalLevel LogLevel = "fatal"
+	PanicLevel LogLevel = "panic"
+)
+
+// LevelsFromString parses a string of level characters and returns a slice of the corresponding log levels
+func LevelsFromString(str string) []LogLevel {
+	levels := make([]LogLevel, 0, len(str))
+	for _, c := range str {
+		switch c {
+		case 'p':
+			levels = append(levels, PanicLevel)
+		case 'f':
+			levels = append(levels, FatalLevel)
+		case 'e':
+			levels = append(levels, ErrorLevel)
+		case 'w':
+			levels = append(levels, WarnLevel)
+		case 'i':
+			levels = append(levels, InfoLevel)
+		case 'd':
+			levels = append(levels, DebugLevel)
+		case 't':
+			levels = append(levels, TraceLevel)
+		default:
+			continue
+		}
+	}
+	return levels
+}
+
+// String returns the log level as a string
+func (level LogLevel) String() string {
+	return string(level)
+}

pkg/notifications/preview/data/preview_strings.go

@@ -0,0 +1,178 @@
+package data
+
+var containerNames = []string{
+	"cyberscribe",
+	"datamatrix",
+	"nexasync",
+	"quantumquill",
+	"aerosphere",
+	"virtuos",
+	"fusionflow",
+	"neuralink",
+	"pixelpulse",
+	"synthwave",
+	"codecraft",
+	"zapzone",
+	"robologic",
+	"dreamstream",
+	"infinisync",
+	"megamesh",
+	"novalink",
+	"xenogenius",
+	"ecosim",
+	"innovault",
+	"techtracer",
+	"fusionforge",
+	"quantumquest",
+	"neuronest",
+	"codefusion",
+	"datadyno",
+	"pixelpioneer",
+	"vortexvision",
+	"cybercraft",
+	"synthsphere",
+	"infinitescript",
+	"roborhythm",
+	"dreamengine",
+	"aquasync",
+	"geniusgrid",
+	"megamind",
+	"novasync-pro",
+	"xenonwave",
+	"ecologic",
+	"innoscan",
+}
+
+var organizationNames = []string{
+	"techwave",
+	"codecrafters",
+	"innotechlabs",
+	"fusionsoft",
+	"cyberpulse",
+	"quantumscribe",
+	"datadynamo",
+	"neuralink",
+	"pixelpro",
+	"synthwizards",
+	"virtucorplabs",
+	"robologic",
+	"dreamstream",
+	"novanest",
+	"megamind",
+	"xenonwave",
+	"ecologic",
+	"innosync",
+	"techgenius",
+	"nexasoft",
+	"codewave",
+	"zapzone",
+	"techsphere",
+	"aquatech",
+	"quantumcraft",
+	"neuronest",
+	"datafusion",
+	"pixelpioneer",
+	"synthsphere",
+	"infinitescribe",
+	"roborhythm",
+	"dreamengine",
+	"vortexvision",
+	"geniusgrid",
+	"megamesh",
+	"novasync",
+	"xenogeniuslabs",
+	"ecosim",
+	"innovault",
+}
+
+var errorMessages = []string{
+	"Error 404: Resource not found",
+	"Critical Error: System meltdown imminent",
+	"Error 500: Internal server error",
+	"Invalid input: Please check your data",
+	"Access denied: Unauthorized access detected",
+	"Network connection lost: Please check your connection",
+	"Error 403: Forbidden access",
+	"Fatal error: System crash imminent",
+	"File not found: Check the file path",
+	"Invalid credentials: Authentication failed",
+	"Error 502: Bad Gateway",
+	"Database connection failed: Please try again later",
+	"Security breach detected: Take immediate action",
+	"Error 400: Bad request",
+	"Out of memory: Close unnecessary applications",
+	"Invalid configuration: Check your settings",
+	"Error 503: Service unavailable",
+	"File is read-only: Cannot modify",
+	"Data corruption detected: Backup your data",
+	"Error 401: Unauthorized",
+	"Disk space full: Free up disk space",
+	"Connection timeout: Retry your request",
+	"Error 504: Gateway timeout",
+	"File access denied: Permission denied",
+	"Unexpected error: Please contact support",
+	"Error 429: Too many requests",
+	"Invalid URL: Check the URL format",
+	"Database query failed: Try again later",
+	"Error 408: Request timeout",
+	"File is in use: Close the file and try again",
+	"Invalid parameter: Check your input",
+	"Error 502: Proxy error",
+	"Database connection lost: Reconnect and try again",
+	"File size exceeds limit: Reduce the file size",
+	"Error 503: Overloaded server",
+	"Operation aborted: Try again",
+	"Invalid API key: Check your API key",
+	"Error 507: Insufficient storage",
+	"Database deadlock: Retry your transaction",
+	"Error 405: Method not allowed",
+	"File format not supported: Choose a different format",
+	"Unknown error: Contact system administrator",
+}
+
+var skippedMessages = []string{
+	"Fear of introducing new bugs",
+	"Don't have time for the update process",
+	"Current version works fine for my needs",
+	"Concerns about compatibility with other software",
+	"Limited bandwidth for downloading updates",
+	"Worries about losing custom settings or configurations",
+	"Lack of trust in the software developer's updates",
+	"Dislike changes to the user interface",
+	"Avoiding potential subscription fees",
+	"Suspicion of hidden data collection in updates",
+	"Apprehension about changes in privacy policies",
+	"Prefer the older version's features or design",
+	"Worry about software becoming more resource-intensive",
+	"Avoiding potential changes in licensing terms",
+	"Waiting for initial bugs to be resolved in the update",
+	"Concerns about update breaking third-party plugins or extensions",
+	"Belief that the software is already secure enough",
+	"Don't want to relearn how to use the software",
+	"Fear of losing access to older file formats",
+	"Avoiding the hassle of having to update multiple devices",
+}
+
+var logMessages = []string{
+	"Checking for available updates...",
+	"Downloading update package...",
+	"Verifying update integrity...",
+	"Preparing to install update...",
+	"Backing up existing configuration...",
+	"Installing update...",
+	"Update installation complete.",
+	"Applying configuration settings...",
+	"Cleaning up temporary files...",
+	"Update successful! Software is now up-to-date.",
+	"Restarting the application...",
+	"Restart complete. Enjoy the latest features!",
+	"Update rollback complete. Your software remains at the previous version.",
+}
+
+var logErrors = []string{
+	"Unable to check for updates. Please check your internet connection.",
+	"Update package download failed. Try again later.",
+	"Update verification failed. Please contact support.",
+	"Update installation failed. Rolling back to the previous version...",
+	"Your configuration settings may have been reset to defaults.",
+}

pkg/notifications/preview/data/report.go

@@ -0,0 +1,110 @@
+package data
+
+import (
+	"sort"
+
+	"github.com/containrrr/watchtower/pkg/types"
+)
+
+// State is the outcome of a container in a session report
+type State string
+
+const (
+	ScannedState State = "scanned"
+	UpdatedState State = "updated"
+	FailedState  State = "failed"
+	SkippedState State = "skipped"
+	StaleState   State = "stale"
+	FreshState   State = "fresh"
+)
+
+// StatesFromString parses a string of state characters and returns a slice of the corresponding report states
+func StatesFromString(str string) []State {
+	states := make([]State, 0, len(str))
+	for _, c := range str {
+		switch c {
+		case 'c':
+			states = append(states, ScannedState)
+		case 'u':
+			states = append(states, UpdatedState)
+		case 'e':
+			states = append(states, FailedState)
+		case 'k':
+			states = append(states, SkippedState)
+		case 't':
+			states = append(states, StaleState)
+		case 'f':
+			states = append(states, FreshState)
+		default:
+			continue
+		}
+	}
+	return states
+}
+
+type report struct {
+	scanned []types.ContainerReport
+	updated []types.ContainerReport
+	failed  []types.ContainerReport
+	skipped []types.ContainerReport
+	stale   []types.ContainerReport
+	fresh   []types.ContainerReport
+}
+
+func (r *report) Scanned() []types.ContainerReport {
+	return r.scanned
+}
+func (r *report) Updated() []types.ContainerReport {
+	return r.updated
+}
+func (r *report) Failed() []types.ContainerReport {
+	return r.failed
+}
+func (r *report) Skipped() []types.ContainerReport {
+	return r.skipped
+}
+func (r *report) Stale() []types.ContainerReport {
+	return r.stale
+}
+func (r *report) Fresh() []types.ContainerReport {
+	return r.fresh
+}
+
+func (r *report) All() []types.ContainerReport {
+	allLen := len(r.scanned) + len(r.updated) + len(r.failed) + len(r.skipped) + len(r.stale) + len(r.fresh)
+	all := make([]types.ContainerReport, 0, allLen)
+
+	presentIds := map[types.ContainerID][]string{}
+
+	appendUnique := func(reports []types.ContainerReport) {
+		for _, cr := range reports {
+			if _, found := presentIds[cr.ID()]; found {
+				continue
+			}
+			all = append(all, cr)
+			presentIds[cr.ID()] = nil
+		}
+	}
+
+	appendUnique(r.updated)
+	appendUnique(r.failed)
+	appendUnique(r.skipped)
+	appendUnique(r.stale)
+	appendUnique(r.fresh)
+	appendUnique(r.scanned)
+
+	sort.Sort(sortableContainers(all))
+
+	return all
+}
+
+type sortableContainers []types.ContainerReport
+
+// Len implements sort.Interface.Len
+func (s sortableContainers) Len() int { return len(s) }
+
+// Less implements sort.Interface.Less
+func (s sortableContainers) Less(i, j int) bool { return s[i].ID() < s[j].ID() }
+
+// Swap implements sort.Interface.Swap
+func (s sortableContainers) Swap(i, j int) { s[i], s[j] = s[j], s[i] }

pkg/notifications/preview/data/status.go

@@ -0,0 +1,44 @@
+package data
+
+import wt "github.com/containrrr/watchtower/pkg/types"
+
+type containerStatus struct {
+	containerID   wt.ContainerID
+	oldImage      wt.ImageID
+	newImage      wt.ImageID
+	containerName string
+	imageName     string
+	error
+	state State
+}
+
+func (u *containerStatus) ID() wt.ContainerID {
+	return u.containerID
+}
+
+func (u *containerStatus) Name() string {
+	return u.containerName
+}
+
+func (u *containerStatus) CurrentImageID() wt.ImageID {
+	return u.oldImage
+}
+
+func (u *containerStatus) LatestImageID() wt.ImageID {
+	return u.newImage
+}
+
+func (u *containerStatus) ImageName() string {
+	return u.imageName
+}
+
+func (u *containerStatus) Error() string {
+	if u.error == nil {
+		return ""
+	}
+	return u.error.Error()
+}
+
+func (u *containerStatus) State() string {
+	return string(u.state)
+}

pkg/notifications/preview/tplprev.go

@@ -0,0 +1,36 @@
+package preview
+
+import (
+	"fmt"
+	"strings"
+	"text/template"
+
+	"github.com/containrrr/watchtower/pkg/notifications/preview/data"
+	"github.com/containrrr/watchtower/pkg/notifications/templates"
+)
+
+func Render(input string, states []data.State, loglevels []data.LogLevel) (string, error) {
+
+	data := data.New()
+
+	tpl, err := template.New("").Funcs(templates.Funcs).Parse(input)
+	if err != nil {
+		return "", fmt.Errorf("failed to parse template: %e", err)
+	}
+
+	for _, state := range states {
+		data.AddFromState(state)
+	}
+
+	for _, level := range loglevels {
+		data.AddLogEntry(level)
+	}
+
+	var buf strings.Builder
+	err = tpl.Execute(&buf, data)
+	if err != nil {
+		return "", fmt.Errorf("failed to execute template: %e", err)
+	}
+
+	return buf.String(), nil
+}

pkg/notifications/shoutrrr.go

@@ -10,10 +10,9 @@ import (
 
 	"github.com/containrrr/shoutrrr"
 	"github.com/containrrr/shoutrrr/pkg/types"
+	"github.com/containrrr/watchtower/pkg/notifications/templates"
 	t "github.com/containrrr/watchtower/pkg/types"
 	log "github.com/sirupsen/logrus"
-	"golang.org/x/text/cases"
-	"golang.org/x/text/language"
 )
 
 // LocalLog is a logrus logger that does not send entries as notifications
@@ -208,13 +207,8 @@ func (n *shoutrrrTypeNotifier) Fire(entry *log.Entry) error {
 }
 
 func getShoutrrrTemplate(tplString string, legacy bool) (tpl *template.Template, err error) {
-	funcs := template.FuncMap{
-		"ToUpper": strings.ToUpper,
-		"ToLower": strings.ToLower,
-		"ToJSON":  toJSON,
-		"Title":   cases.Title(language.AmericanEnglish).String,
-	}
-	tplBase := template.New("").Funcs(funcs)
+
+	tplBase := template.New("").Funcs(templates.Funcs)
 
 	if builtin, found := commonTemplates[tplString]; found {
 		log.WithField(`template`, tplString).Debug(`Using common template`)

pkg/notifications/templates/funcs.go

@@ -0,0 +1,27 @@
+package templates
+
+import (
+	"encoding/json"
+	"fmt"
+	"strings"
+	"text/template"
+
+	"golang.org/x/text/cases"
+	"golang.org/x/text/language"
+)
+
+var Funcs = template.FuncMap{
+	"ToUpper": strings.ToUpper,
+	"ToLower": strings.ToLower,
+	"ToJSON":  toJSON,
+	"Title":   cases.Title(language.AmericanEnglish).String,
+}
+
+func toJSON(v interface{}) string {
+	var bytes []byte
+	var err error
+	if bytes, err = json.MarshalIndent(v, "", "  "); err != nil {
+		return fmt.Sprintf("failed to marshal JSON in notification template: %v", err)
+	}
+	return string(bytes)
+}

scripts/build-tplprev.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+cd $(git rev-parse --show-toplevel)
+
+cp "$(go env GOROOT)/misc/wasm/wasm_exec.js" ./docs/assets/
+
+GOARCH=wasm GOOS=js go build -o ./docs/assets/tplprev.wasm ./tplprev

tplprev/main.go

@@ -0,0 +1,49 @@
+//go:build !wasm
+
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+
+	"github.com/containrrr/watchtower/internal/meta"
+	"github.com/containrrr/watchtower/pkg/notifications/preview"
+	"github.com/containrrr/watchtower/pkg/notifications/preview/data"
+)
+
+func main() {
+	fmt.Fprintf(os.Stderr, "watchtower/tplprev %v\n\n", meta.Version)
+
+	var states string
+	var entries string
+
+	flag.StringVar(&states, "states", "cccuuueeekkktttfff", "sCanned, Updated, failEd, sKipped, sTale, Fresh")
+	flag.StringVar(&entries, "entries", "ewwiiidddd", "Fatal,Error,Warn,Info,Debug,Trace")
+
+	flag.Parse()
+
+	if len(flag.Args()) < 1 {
+		fmt.Fprintln(os.Stderr, "Missing required argument TEMPLATE")
+		flag.Usage()
+		os.Exit(1)
+		return
+	}
+
+	input, err := os.ReadFile(flag.Arg(0))
+	if err != nil {
+
+		fmt.Fprintf(os.Stderr, "Failed to read template file %q: %v\n", flag.Arg(0), err)
+		os.Exit(1)
+		return
+	}
+
+	result, err := preview.Render(string(input), data.StatesFromString(states), data.LevelsFromString(entries))
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Failed to read template file %q: %v\n", flag.Arg(0), err)
+		os.Exit(1)
+		return
+	}
+
+	fmt.Println(result)
+}

tplprev/main_wasm.go

@@ -0,0 +1,62 @@
+//go:build wasm
+
+package main
+
+import (
+	"fmt"
+
+	"github.com/containrrr/watchtower/internal/meta"
+	"github.com/containrrr/watchtower/pkg/notifications/preview"
+	"github.com/containrrr/watchtower/pkg/notifications/preview/data"
+
+	"syscall/js"
+)
+
+func main() {
+	fmt.Println("watchtower/tplprev v" + meta.Version)
+
+	js.Global().Set("WATCHTOWER", js.ValueOf(map[string]any{
+		"tplprev": js.FuncOf(jsTplPrev),
+	}))
+	<-make(chan bool)
+
+}
+
+func jsTplPrev(this js.Value, args []js.Value) any {
+
+	if len(args) < 3 {
+		return "Requires 3 arguments passed"
+	}
+
+	input := args[0].String()
+
+	statesArg := args[1]
+	var states []data.State
+
+	if statesArg.Type() == js.TypeString {
+		states = data.StatesFromString(statesArg.String())
+	} else {
+		for i := 0; i < statesArg.Length(); i++ {
+			state := data.State(statesArg.Index(i).String())
+			states = append(states, state)
+		}
+	}
+
+	levelsArg := args[2]
+	var levels []data.LogLevel
+
+	if levelsArg.Type() == js.TypeString {
+		levels = data.LevelsFromString(statesArg.String())
+	} else {
+		for i := 0; i < levelsArg.Length(); i++ {
+			level := data.LogLevel(levelsArg.Index(i).String())
+			levels = append(levels, level)
+		}
+	}
+
+	result, err := preview.Render(input, states, levels)
+	if err != nil {
+		return "Error: " + err.Error()
+	}
+	return result
+}