Template Functions

Sparrow provides 22 built-in template functions for transforming webhook payloads. These are available in subscription transform_template fields.

Templates use Go’s text/template syntax. The event payload is available as the template data context (e.g., {{ .payload.field_name }}).

Quick Reference

Function	Description
json	Convert value to JSON string
urlencode	URL-encode a string
base64	Base64-encode a string
base64decode	Base64-decode a string
now	Current time
formatTime	Format a time value
parseTime	Parse a time string
upper	Uppercase string
lower	Lowercase string
title	Title case (all uppercase)
trim	Trim characters from both ends
trimSpace	Trim whitespace
split	Split string into slice
join	Join slice with separator
default	Default value for nil/empty
slice	Substring by index
len	Length of string/slice/map
truncate	Truncate string (hard cut)
ellipsis	Truncate with ”…” suffix
repeat	Repeat string N times
contains	Check if string contains substring
hasPrefix	Check string prefix
hasSuffix	Check string suffix
replace	Replace all occurrences

---

json

Converts any value to a JSON string.

{{ .data | json }}
{{ json .payload }}

Example:

Input:  map[string]any{"name": "John", "age": 30}
Output: {"name":"John","age":30}

---

urlencode

URL-encodes a string by escaping special characters.

{{ .email | urlencode }}
{{ urlencode "hello world" }}

Example:

Input:  "hello world@example.com"
Output: "hello+world%40example.com"

---

base64

Base64-encodes a string.

{{ .secret | base64 }}
{{ base64 "hello world" }}

Example:

Input:  "hello world"
Output: "aGVsbG8gd29ybGQ="

---

base64decode

Base64-decodes a string back to its original form.

{{ .encodedSecret | base64decode }}
{{ base64decode "aGVsbG8gd29ybGQ=" }}

Example:

Input:  "aGVsbG8gd29ybGQ="
Output: "hello world"

---

now

Returns the current time as a time.Time object.

{{ now }}
{{ now | formatTime "2006-01-02 15:04:05" }}

Example:

Output: 2023-11-21 14:30:45 +0000 UTC

---

formatTime

Formats a time value using Go’s time layout format.

{{ formatTime "2006-01-02" .createdAt }}
{{ .timestamp | formatTime "15:04:05" }}

Common layouts:

• RFC3339: 2006-01-02T15:04:05Z07:00
• Date only: 2006-01-02
• Time only: 15:04:05
• Human readable: January 2, 2006 at 3:04 PM

---

parseTime

Parses a time string using Go’s time layout format.

{{ parseTime "2006-01-02" "2023-11-21" }}
{{ parseTime "15:04:05" .timeString }}

Example:

Input:  "2006-01-02", "2023-11-21"
Output: 2023-11-21 00:00:00 +0000 UTC

---

upper

Converts string to uppercase.

{{ .name | upper }}
{{ upper "hello world" }}

Example: "hello world" -> "HELLO WORLD"

---

lower

Converts string to lowercase.

{{ .name | lower }}
{{ lower "HELLO WORLD" }}

Example: "HELLO WORLD" -> "hello world"

---

title

Converts all letters to uppercase (note: not proper title case).

{{ .name | title }}
{{ title "hello world" }}

Example: "hello world" -> "HELLO WORLD"

---

trim

Trims specified characters from both ends of a string.

{{ trim " " .text }}
{{ trim "." "...hello..." }}

Example:

Input:  " ", "  hello world  "
Output: "hello world"

---

trimSpace

Trims whitespace (spaces, tabs, newlines) from both ends.

{{ .userInput | trimSpace }}
{{ trimSpace "  hello world  " }}

Example: " hello world \n" -> "hello world"

---

split

Splits a string by separator into a slice.

{{ split "," "apple,banana,cherry" }}
{{ .tags | split "|" }}

Use with range to iterate:

{{range split "," .tags}}
- {{.}}
{{end}}

---

join

Joins a string slice with a separator.

{{ join ", " .tags }}
{{ .items | join " | " }}

Example: ["apple", "banana", "cherry"] -> "apple, banana, cherry"

---

default

Returns a default value if the input is nil or empty string.

{{ .optionalField | default "N/A" }}
{{ default "Unknown" .name }}

Example:

Input:  "N/A", ""      -> "N/A"
Input:  "N/A", "John"  -> "John"

---

slice

Returns a substring from start to end index (end exclusive). Use -1 for end of string.

{{ slice 0 5 "hello world" }}
{{ .text | slice 2 8 }}

Example:

Input:  0, 5, "hello world"  -> "hello"
Input:  6, -1, "hello world" -> "world"

---

len

Returns the length of a string, slice, or map.

{{ len .name }}
{{ .items | len }}

---

truncate

Truncates a string to a maximum length (hard cut, no ellipsis).

{{ truncate 10 .longText }}
{{ .description | truncate 50 }}

Example: truncate 10 "this is a very long string" -> "this is a "

---

ellipsis

Truncates a string to a maximum length and adds ... if truncated.

{{ ellipsis 20 .longText }}
{{ .description | ellipsis 100 }}

Example:

ellipsis 20 "this is a very long string that needs truncation"
-> "this is a very lo..."

ellipsis 10 "short"
-> "short"

If maxLen <= 3, no ellipsis is added.

---

repeat

Repeats a string a specified number of times.

{{ repeat 3 "*" }}
{{ .pattern | repeat 5 }}

Example: repeat 3 "*" -> "***"

---

contains

Checks if a string contains a substring (case-sensitive).

{{ if contains "error" .message }}
  Error found!
{{ end }}

---

hasPrefix

Checks if a string starts with a prefix (case-sensitive).

{{ if hasPrefix "http" .url }}
  Valid URL
{{ end }}

---

hasSuffix

Checks if a string ends with a suffix (case-sensitive).

{{ if hasSuffix ".jpg" .filename }}
  Image file
{{ end }}

---

replace

Replaces all occurrences of a substring with another.

{{ replace " " "_" .name }}
{{ .text | replace "foo" "bar" }}

Example: replace " " "_" "hello world test" -> "hello_world_test"