# Template filters Format text, prices, dates, and collections directly in your Go templates. Learn the calling conventions first, then use the reference below to find every registered filter. ## Available in every Go template {#overview} HyperChi copies `filters.FuncMap()` when you create an application. Templates loaded with `app.LoadTemplates` can call these functions immediately. They are Go template functions, not a separate expression language. The external templ and Jet integrations do not inherit this function map. ```go app := hyperchi.New() helpers.Must(app.LoadTemplates("templates/*.tmpl")) app.Get("/product", func(c *hyperchi.Context) error { return c.View("product.tmpl", hyperchi.H{ "name": " field notebook ", "price": 24.5, "published": "2026-10-05T12:00:00Z", }) }) ``` ```html

{{ .name | trim | title }}

{{ currency .price "$" }}

``` The page displays “Field Notebook”, “$24.50”, and “Oct 5, 2026”. The handler uses only the core `hyperchi` package. `c.View` hands the values to the template unchanged, so a `time.Time` works as well as an RFC 3339 string. ## Argument order matters {#arguments} A Go pipeline places its result in the **last** argument. Unary functions work naturally in a chain. Most multi-argument HyperChi filters take the value **first**, so use a direct call or parentheses. ```html {{ .name | trim | upper }} {{ truncate .name 20 }} {{ currency .price "€" }} {{ default .nickname "Anonymous" }} {{ join (sort .tags) ", " }} {{ .published | dateFormat "2006-01-02" }} ``` > `{{ .name | truncate 20 }}` is not equivalent to `{{ truncate .name 20 }}`. It sends `20` as the first argument. Likewise, `default` takes the value before the fallback. `dateFormat` intentionally takes the format first and supports the pipeline shown above. ## Recipes with live output {#example} These outputs are rendered by this page using the installed filters. Change the literals in your own templates to adapt them. ### Clean a label ```gotemplate {{ " field notebook " | trim | title }} ``` Result: `Field Notebook` ### Shorten text ```gotemplate {{ truncate "A notebook for every journey" 18 }} ``` Result: `A notebook for ...` ### Format a price ```gotemplate {{ currency 1234.5 "$" }} ``` Result: `$1,234.50` ### Display a proportion ```gotemplate {{ percent 0.125 1 }} ``` Result: `12.5%` ### Human-readable storage ```gotemplate {{ bytes 1536 }} ``` Result: `1.50 KB` ### Format an ISO date ```gotemplate {{ "2026-10-05T12:00:00Z" | dateFormat "Jan 2, 2006" }} ``` Result: `Oct 5, 2026` ### Use a fallback ```gotemplate {{ default "" "Anonymous" }} ``` Result: `Anonymous` ### Sort a collection ```gotemplate {{ range sort (list "htmx" "go" "html") }}{{ . }} {{ end }} ``` Result: `go html htmx` ### Read nested JSON ```gotemplate {{ jsonGet (fromJSON `{"user":{"name":"Morgan"}}`) "user.name" }} ``` Result: `Morgan` ### Count items ```gotemplate {{ len (list "Go" "HTML" "HTMX") }} ``` Result: `3` ## Work with collections {#collections} `sortBy`, `filter`, and `pluck` accept a collection followed by a field name. Use map keys for maps such as `hyperchi.H`. For structs, use the exported Go field name. Sorting uses string representations, including in `sortBy`; sort numeric values in Go when numeric order matters. Keep database queries and business decisions in the handler. ```html {{ $active := filter .products "active" true }} {{ range take (sortBy $active "name") 3 }}

{{ .name }}

{{ currency .price }}

{{ else }}

No matching products.

{{ end }} ``` `contains` is a substring test for strings and a membership test for slices and arrays (by the rules of `eq`, so `2` matches `2.0`). `join` expects `[]string`, while many collection transforms return `[]any`. Render those results with `range` instead of passing them to `join`. ## HTML, JSON, and htmx {#html} Go’s `html/template` escapes ordinary values for their HTML context. Render visitor text directly; adding `escape` can double-escape it. `nl2br` escapes its text before inserting line breaks. ```html

{{ .message }}

{{ nl2br .message }}
Settings
``` Load htmx separately to activate these attributes. Attribute helpers generate individual attributes: place them side by side, not in a pipeline. Keep attribute names, event expressions, and destinations application-controlled. `safe`, `unescape`, `css`, and `js` mark content as trusted; they do not sanitize it. `stripTags` is a regular-expression transform that also returns trusted HTML, not a security boundary. Only use trusted-content helpers after an appropriate sanitization or application-owned-content boundary. `htmxButton` and `htmxForm` return complete elements, not attributes. They take typed `filters.HTMXButtonOptions` and `filters.HTMXFormOptions`, not a `dict`. Pass the typed options in the handler data, or use literal elements with individual helpers, as above. ## Register a custom filter {#custom} Register filters during startup, before loading templates and before serving traffic. Use a distinctive name so you do not accidentally replace a built-in function. ```go app.RegisterFilter("excerpt", func(limit int, text string) string { if limit < 0 { limit = 0 } runes := []rune(text) if len(runes) > limit { return string(runes[:limit]) + "…" } return text }) helpers.Must(app.LoadTemplates("templates/*.tmpl")) ``` ```html {{ .description | excerpt 80 }} ``` This custom function deliberately puts text last for pipeline use. Register several functions with `app.RegisterFilters(template.FuncMap{...})`. To use the built-ins outside HyperChi, call `template.New("page").Funcs(filters.FuncMap()).Parse(source)`. Package-level `filters.Register` affects future snapshots; it does not update applications that already copied their function maps. ## Types and failure behavior {#errors} Functions returning `(value, error)`, such as `fromJSON` and `dateAdd`, stop template execution on a non-nil error. Other helpers return fallback values: an unparseable `date` produces an empty string, while an unconvertible `currency` value is printed unchanged. Validate input in the handler rather than treating formatting as validation. `default` treats false, numeric zero, and empty collections as empty, as well as empty strings and nil. Use `coalesce` when only nil should trigger a fallback. Validate sizes and indexes before passing visitor-controlled numbers to collection or truncation helpers. ## Complete registered reference {#reference} Signatures below come from the running filter registry. Arguments appear in call order; `...` means variadic arguments and `interface {}` means `any`. Expand a category to inspect every function. Functions requiring typed options or callbacks are generally easier to use from Go with direct template execution. ### Strings (31 functions) - `br2nl`: `func(string) string` - `camelCase`: `func(string) string` - `capitalize`: `func(string) string` - `dedent`: `func(string) string` - `hasPrefix`: `func(string, string) bool` - `hasSuffix`: `func(string, string) bool` - `indent`: `func(string, int, ...string) string` - `join`: `func([]string, string) string` - `kebabCase`: `func(string) string` - `lower`: `func(string) string` - `nl2br`: `func(string) template.HTML` - `pascalCase`: `func(string) string` - `pluralize`: `func(string, ...int) string` - `quote`: `func(string) string` - `repeat`: `func(string, int) string` - `replace`: `func(string, string, string, int) string` - `replaceAll`: `func(string, string, string) string` - `singularize`: `func(string) string` - `slug`: `func(string) string` - `snakeCase`: `func(string) string` - `split`: `func(string, string) []string` - `stripTags`: `func(string) template.HTML` - `title`: `func(string) string` - `trim`: `func(string) string` - `trimPrefix`: `func(string, string) string` - `trimSuffix`: `func(string, string) string` - `truncate`: `func(string, int, ...string) string` - `truncateWords`: `func(string, int, ...string) string` - `unquote`: `func(string) string` - `upper`: `func(string) string` - `wordWrap`: `func(string, int) string` ### HTML (27 functions) - `addClass`: `func(string, string) string` - `ariaAttrs`: `func(map[string]interface {}) (template.HTMLAttr, error)` - `attr`: `func(string, string) (template.HTMLAttr, error)` - `checkbox`: `func(string, interface {}, bool, ...template.HTMLAttr) template.HTML` - `classNames`: `func(...interface {}) string` - `css`: `func(string) template.CSS` - `dataAttrs`: `func(map[string]interface {}) (template.HTMLAttr, error)` - `escape`: `func(string) string` - `formField`: `func(string, string, interface {}, ...template.HTMLAttr) template.HTML` - `img`: `func(string, string, ...template.HTMLAttr) template.HTML` - `inputAttrs`: `func(filters.InputOptions) (template.HTMLAttr, error)` - `js`: `func(string) template.JS` - `link`: `func(string, string, ...template.HTMLAttr) template.HTML` - `mailto`: `func(string, ...string) template.HTML` - `meta`: `func(string, string) template.HTML` - `radio`: `func(string, interface {}, bool, ...template.HTMLAttr) template.HTML` - `removeClass`: `func(string, string) string` - `safe`: `func(string) template.HTML` - `script`: `func(string, ...template.HTMLAttr) template.HTML` - `select`: `func(string, []filters.SelectOption, ...template.HTMLAttr) template.HTML` - `style`: `func(string, ...template.HTMLAttr) template.HTML` - `tel`: `func(string, ...string) template.HTML` - `textarea`: `func(string, interface {}, int, int, ...template.HTMLAttr) template.HTML` - `toggleClass`: `func(string, string) string` - `unescape`: `func(string) template.HTML` - `url`: `func(string) string` - `urlquery`: `func(string) string` ### JSON (12 functions) - `compactJSON`: `func(string) (string, error)` - `fromJSON`: `func(string) (interface {}, error)` - `jsonArray`: `func(...interface {}) []interface {}` - `jsonAttr`: `func(string, interface {}) (template.HTMLAttr, error)` - `jsonEscape`: `func(string) string` - `jsonGet`: `func(interface {}, string) interface {}` - `jsonLD`: `func(interface {}) template.HTML` - `jsonMerge`: `func(interface {}, interface {}) interface {}` - `jsonObject`: `func(...interface {}) map[string]interface {}` - `jsonSet`: `func(interface {}, string, interface {}) interface {}` - `prettyJSON`: `func(interface {}) (string, error)` - `toJSON`: `func(interface {}) (string, error)` ### Dates and times (26 functions) - `date`: `func(interface {}, ...string) string` - `dateAdd`: `func(interface {}, int, string) (time.Time, error)` - `dateCompare`: `func(interface {}, interface {}) (int, error)` - `dateDiff`: `func(interface {}, interface {}, string) (int64, error)` - `dateFormat`: `func(string, interface {}) string` - `dateSub`: `func(interface {}, int, string) (time.Time, error)` - `day`: `func(interface {}) int` - `durationSeconds`: `func(time.Duration) float64` - `fromUnix`: `func(int64) time.Time` - `hour`: `func(interface {}) int` - `humanDuration`: `func(interface {}) string` - `iso8601`: `func(interface {}) string` - `minute`: `func(interface {}) int` - `month`: `func(interface {}) string` - `now`: `func() time.Time` - `rfc3339`: `func(interface {}) string` - `second`: `func(interface {}) int` - `timeAgo`: `func(interface {}) string` - `timeFrom`: `func(interface {}) string` - `timezone`: `func(interface {}, string) (time.Time, error)` - `today`: `func() time.Time` - `tomorrow`: `func() time.Time` - `unix`: `func(interface {}) int64` - `weekday`: `func(interface {}) string` - `year`: `func(interface {}) int` - `yesterday`: `func() time.Time` ### Numbers (25 functions) - `abs`: `func(interface {}) float64` - `add`: `func(interface {}, interface {}) float64` - `avg`: `func(...interface {}) float64` - `bytes`: `func(interface {}) string` - `ceil`: `func(interface {}) float64` - `commas`: `func(string) string` - `currency`: `func(interface {}, ...string) string` - `decimal`: `func(interface {}, int) string` - `div`: `func(interface {}, interface {}) float64` - `floor`: `func(interface {}) float64` - `humanize`: `func(interface {}) string` - `max`: `func(...interface {}) float64` - `min`: `func(...interface {}) float64` - `mod`: `func(interface {}, interface {}) float64` - `mul`: `func(interface {}, interface {}) float64` - `number`: `func(interface {}, ...int) string` - `ordinal`: `func(interface {}) string` - `padNumber`: `func(interface {}, int, ...string) string` - `percent`: `func(interface {}, ...int) string` - `pow`: `func(interface {}, interface {}) float64` - `roman`: `func(interface {}) string` - `round`: `func(interface {}, ...int) float64` - `sqrt`: `func(interface {}) float64` - `sub`: `func(interface {}, interface {}) float64` - `sum`: `func(...interface {}) float64` ### Collections (28 functions) - `chunk`: `func(interface {}, int) [][]interface {}` - `contains`: `func(interface {}, interface {}) bool` - `filter`: `func(interface {}, string, interface {}) []interface {}` - `find`: `func(interface {}, string, interface {}) interface {}` - `findIndex`: `func(interface {}, string, interface {}) int` - `first`: `func(interface {}) interface {}` - `flatten`: `func(interface {}) []interface {}` - `group`: `func(interface {}, string) map[string][]interface {}` - `index`: `func(interface {}, int) interface {}` - `keys`: `func(interface {}) []string` - `last`: `func(interface {}) interface {}` - `len`: `func(interface {}) int` - `map`: `func(interface {}, string) []interface {}` - `merge`: `func(...interface {}) map[string]interface {}` - `pairs`: `func(interface {}) [][]interface {}` - `pluck`: `func(interface {}, string) []interface {}` - `random`: `func(interface {}) interface {}` - `reduce`: `func(interface {}, func(interface {}, interface {}) interface {}, interface {}) interface {}` - `reverse`: `func(interface {}) []interface {}` - `shuffle`: `func(interface {}) []interface {}` - `skip`: `func(interface {}, int) []interface {}` - `slice`: `func(interface {}, int, ...int) []interface {}` - `sort`: `func(interface {}) []interface {}` - `sortBy`: `func(interface {}, string) []interface {}` - `take`: `func(interface {}, int) []interface {}` - `unique`: `func(interface {}) []interface {}` - `values`: `func(interface {}) []interface {}` - `zip`: `func(...interface {}) [][]interface {}` ### HTMX (39 functions) - `htmxAttrs`: `func(filters.HTMXAttrs) (template.HTMLAttr, error)` - `htmxButton`: `func(filters.HTMXButtonOptions) (template.HTML, error)` - `htmxForm`: `func(filters.HTMXFormOptions) (template.HTML, error)` - `htmxLink`: `func(string, string, ...map[string]string) (template.HTML, error)` - `htmxModal`: `func(string, template.HTML) template.HTML` - `htmxSpinner`: `func(string) template.HTML` - `htmxToast`: `func(string, string) template.HTML` - `hxAction`: `func(string) template.HTMLAttr` - `hxBoost`: `func(bool) template.HTMLAttr` - `hxConfig`: `func(map[string]interface {}) template.HTMLAttr` - `hxConfirm`: `func(string) template.HTMLAttr` - `hxDelete`: `func(string) template.HTMLAttr` - `hxDisabledElt`: `func(string) template.HTMLAttr` - `hxEncoding`: `func(string) template.HTMLAttr` - `hxGet`: `func(string) template.HTMLAttr` - `hxHeaders`: `func(map[string]string) template.HTMLAttr` - `hxIgnore`: `func() template.HTMLAttr` - `hxInclude`: `func(string) template.HTMLAttr` - `hxIndicator`: `func(string) template.HTMLAttr` - `hxMethod`: `func(string) template.HTMLAttr` - `hxOn`: `func(string, string) (template.HTMLAttr, error)` - `hxOptimistic`: `func(string) template.HTMLAttr` - `hxPatch`: `func(string) template.HTMLAttr` - `hxPost`: `func(string) template.HTMLAttr` - `hxPreload`: `func(string) template.HTMLAttr` - `hxPreserve`: `func(interface {}) template.HTMLAttr` - `hxPushURL`: `func(string) template.HTMLAttr` - `hxPut`: `func(string) template.HTMLAttr` - `hxReplaceURL`: `func(interface {}) template.HTMLAttr` - `hxSelect`: `func(string) template.HTMLAttr` - `hxSelectOOB`: `func(string) template.HTMLAttr` - `hxStatus`: `func(string, string) (template.HTMLAttr, error)` - `hxSwap`: `func(string) template.HTMLAttr` - `hxSwapOOB`: `func(string) template.HTMLAttr` - `hxSync`: `func(string) template.HTMLAttr` - `hxTarget`: `func(string) template.HTMLAttr` - `hxTrigger`: `func(string) template.HTMLAttr` - `hxValidate`: `func(bool) template.HTMLAttr` - `hxVals`: `func(map[string]interface {}) template.HTMLAttr` ### Utilities (37 functions) - `and`: `func(...interface {}) bool` - `base64Decode`: `func(string) (string, error)` - `base64Encode`: `func(string) string` - `coalesce`: `func(...interface {}) interface {}` - `default`: `func(interface {}, interface {}) interface {}` - `dict`: `func(...interface {}) map[string]interface {}` - `empty`: `func(interface {}) bool` - `env`: `func(string, ...string) string` - `eq`: `func(interface {}, ...interface {}) bool` - `fallback`: `func(...interface {}) interface {}` - `ge`: `func(interface {}, interface {}) bool` - `gt`: `func(interface {}, interface {}) bool` - `isNil`: `func(interface {}) bool` - `kindOf`: `func(interface {}) string` - `le`: `func(interface {}, interface {}) bool` - `list`: `func(...interface {}) []interface {}` - `lt`: `func(interface {}, interface {}) bool` - `ne`: `func(interface {}, interface {}) bool` - `not`: `func(interface {}) bool` - `notEmpty`: `func(interface {}) bool` - `notNil`: `func(interface {}) bool` - `or`: `func(...interface {}) bool` - `pathBase`: `func(string) string` - `pathDir`: `func(string) string` - `pathExt`: `func(string) string` - `pathJoin`: `func(...string) string` - `print`: `func(...interface {}) string` - `printf`: `func(string, ...interface {}) string` - `println`: `func(...interface {}) string` - `regexMatch`: `func(string, string) (bool, error)` - `regexReplace`: `func(string, string, string) (string, error)` - `regexSplit`: `func(string, string) ([]string, error)` - `ternary`: `func(bool, interface {}, interface {}) interface {}` - `typeOf`: `func(interface {}) string` - `urlDecode`: `func(string) (string, error)` - `urlEncode`: `func(string) string` - `uuid`: `func() (string, error)` ## Keep exploring {#next} Read [Templates](/docs/templates) for composition, [Passing data](/docs/data) for view models, or the [filter implementations and tests](https://github.com/regiellis/hyperchi/tree/main/hyperchi/filters) for detailed edge behavior. Formatting helpers do not automatically transfer to [templ](/docs/templ) or [Jet](/docs/jet); those renderers own their data and helper APIs.