Overview

Package template (html/template) implements data-driven templates for generating
HTML output safe against code injection. It provides the same interface as
package text/template and should be used instead of text/template whenever the
output is HTML.

The documentation here focuses on the security features of the package. For
information about how to program the templates themselves, see the documentation
for text/template.

Introduction

This package wraps package text/template so you can share its template API to
parse and execute HTML templates safely.

If successful, tmpl will now be injection-safe. Otherwise, err is an error
defined in the docs for ErrorCode.

HTML templates treat data values as plain text which should be encoded so they
can be safely embedded in an HTML document. The escaping is contextual, so
actions can appear within JavaScript, CSS, and URI contexts.

The security model used by this package assumes that template authors are
trusted, while Execute’s data parameter is not. More details are provided below.

Example

  1. import "text/template"
  2. ...
  3. t, err := template.New("foo").Parse(`{{define "T"}}Hello, {{.}}!{{end}}`)
  4. err = t.ExecuteTemplate(out, "T", "<script>alert('you have been pwned')</script>")

produces

  1. Hello, <script>alert('you have been pwned')</script>!

but the contextual autoescaping in html/template

  1. import "html/template"
  2. ...
  3. t, err := template.New("foo").Parse(`{{define "T"}}Hello, {{.}}!{{end}}`)
  4. err = t.ExecuteTemplate(out, "T", "<script>alert('you have been pwned')</script>")

produces safe, escaped HTML output

  1. Hello, &lt;script&gt;alert(&#39;you have been pwned&#39;)&lt;/script&gt;!

Contexts

This package understands HTML, CSS, JavaScript, and URIs. It adds sanitizing
functions to each simple action pipeline, so given the excerpt

  1. <a href="/search?q={{.}}">{{.}}</a>

At parse time each {{.}} is overwritten to add escaping functions as necessary.
In this case it becomes

  1. <a href="/search?q={{. | urlescaper | attrescaper}}">{{. | htmlescaper}}</a>

where urlescaper, attrescaper, and htmlescaper are aliases for internal escaping
functions.

Errors

See the documentation of ErrorCode for details.

A fuller picture

The rest of this package comment may be skipped on first reading; it includes
details necessary to understand escaping contexts and error messages. Most users
will not need to understand these details.

Contexts

Assuming {{.}} is O'Reilly: How are <i>you</i>?, the table below shows how
{{.}} appears when used in the context to the left.

  1. Context                          {{.}} After
  2. {{.}}                            O'Reilly: How are &lt;i&gt;you&lt;/i&gt;?
  3. <a title='{{.}}'>                O&#39;Reilly: How are you?
  4. <a href="/{{.}}">                O&#39;Reilly: How are %3ci%3eyou%3c/i%3e?
  5. <a href="?q={{.}}">              O&#39;Reilly%3a%20How%20are%3ci%3e...%3f
  6. <a onx='f("{{.}}")'>             O\x27Reilly: How are \x3ci\x3eyou...?
  7. <a onx='f({{.}})'>               "O\x27Reilly: How are \x3ci\x3eyou...?"
  8. <a onx='pattern = /{{.}}/;'>     O\x27Reilly: How are \x3ci\x3eyou...\x3f

If used in an unsafe context, then the value might be filtered out:

  1. Context                          {{.}} After
  2. <a href="{{.}}">                 #ZgotmplZ

since “O’Reilly:” is not an allowed protocol like “http:”.

If {{.}} is the innocuous word, left, then it can appear more widely,

  1. Context                              {{.}} After
  2. {{.}}                                left
  3. <a title='{{.}}'>                    left
  4. <a href='{{.}}'>                     left
  5. <a href='/{{.}}'>                    left
  6. <a href='?dir={{.}}'>                left
  7. <a style="border-{{.}}: 4px">        left
  8. <a style="align: {{.}}">             left
  9. <a style="background: '{{.}}'>       left
  10. <a style="background: url('{{.}}')>  left
  11. <style>p.{{.}} {color:red}</style>   left

Non-string values can be used in JavaScript contexts. If {{.}} is

  1. struct{A,B string}{ "foo", "bar" }

in the escaped template

  1. <script>var pair = {{.}};</script>

then the template output is

  1. <script>var pair = {"A": "foo", "B": "bar"};</script>

See package json to understand how non-string content is marshaled for embedding
in JavaScript contexts.

Typed Strings

By default, this package assumes that all pipelines produce a plain text string.
It adds escaping pipeline stages necessary to correctly and safely embed that
plain text string in the appropriate context.

When a data value is not plain text, you can make sure it is not over-escaped by
marking it with its type.

Types HTML, JS, URL, and others from content.go can carry safe content that is
exempted from escaping.

The template

  1. Hello, {{.}}!

can be invoked with

  1. tmpl.Execute(out, template.HTML(`<b>World</b>`))

to produce

  1. Hello, <b>World</b>!

instead of the

  1. Hello, &lt;b&gt;World&lt;b&gt;!

that would have been produced if {{.}} was a regular string.

Security Model

https://rawgit.com/mikesamuel/sanitized-jquery-templates/trunk/safetemplate.html#problem_definition
defines “safe” as used by this package.

This package assumes that template authors are trusted, that Execute’s data
parameter is not, and seeks to preserve the properties below in the face of
untrusted data:

Structure Preservation Property: “… when a template author writes an HTML tag
in a safe templating language, the browser will interpret the corresponding
portion of the output as a tag regardless of the values of untrusted data, and
similarly for other structures such as attribute boundaries and JS and CSS
string boundaries.”

Code Effect Property: “… only code specified by the template author should run
as a result of injecting the template output into a page and all code specified
by the template author should run as a result of the same.”

Least Surprise Property: “A developer (or code reviewer) familiar with HTML,
CSS, and JavaScript, who knows that contextual autoescaping happens should be
able to look at a {{.}} and correctly infer what sanitization happens.”


Example:

  1. const tpl = `
  2. <!DOCTYPE html>
  3. <html>
  4.     <head>
  5.         <meta charset="UTF-8">
  6.         <title>{{.Title}}</title>
  7.     </head>
  8.     <body>
  9.         {{range .Items}}<div>{{ . }}</div>{{else}}<div><strong>no rows</strong></div>{{end}}
  10.     </body>
  11. </html>`
  13. check := func(err error) {
  14.     if err != nil {
  15.         log.Fatal(err)
  16.     }
  17. }
  18. t, err := template.New("webpage").Parse(tpl)
  19. check(err)
  21. data := struct {
  22.     Title string
  23.     Items []string
  24. }{
  25.     Title: "My page",
  26.     Items: []string{
  27.         "My photos",
  28.         "My blog",
  29.     },
  30. }
  32. err = t.Execute(os.Stdout, data)
  33. check(err)
  35. noItems := struct {
  36.     Title string
  37.     Items []string
  38. }{
  39.     Title: "My another page",
  40.     Items: []string{},
  41. }
  43. err = t.Execute(os.Stdout, noItems)
  44. check(err)
  46. // Output:
  47. // <!DOCTYPE html>
  48. // <html>
  49. //     <head>
  50. //         <meta charset="UTF-8">
  51. //         <title>My page</title>
  52. //     </head>
  53. //     <body>
  54. //         <div>My photos</div><div>My blog</div>
  55. //     </body>
  56. // </html>
  57. // <!DOCTYPE html>
  58. // <html>
  59. //     <head>
  60. //         <meta charset="UTF-8">
  61. //         <title>My another page</title>
  62. //     </head>
  63. //     <body>
  65. //     </body>
  66. // </html>


Example:

  1. check := func(err error) {
  2.     if err != nil {
  3.         log.Fatal(err)
  4.     }
  5. }
  6. t, err := template.New("foo").Parse(`{{define "T"}}Hello, {{.}}!{{end}}`)
  7. check(err)
  8. err = t.ExecuteTemplate(os.Stdout, "T", "<script>alert('you have been pwned')</script>")
  9. check(err)
  10. // Output:
  11. // Hello, &lt;script&gt;alert(&#39;you have been pwned&#39;)&lt;/script&gt;!


Example:

  1. const s = `"Fran & Freddie's Diner" <tasty@example.com>`
  2. v := []interface{}{`"Fran & Freddie's Diner"`, ' ', `<tasty@example.com>`}
  4. fmt.Println(template.HTMLEscapeString(s))
  5. template.HTMLEscape(os.Stdout, []byte(s))
  6. fmt.Fprintln(os.Stdout, "")
  7. fmt.Println(template.HTMLEscaper(v...))
  9. fmt.Println(template.JSEscapeString(s))
  10. template.JSEscape(os.Stdout, []byte(s))
  11. fmt.Fprintln(os.Stdout, "")
  12. fmt.Println(template.JSEscaper(v...))
  14. fmt.Println(template.URLQueryEscaper(v...))
  16. // Output:
  17. // &#34;Fran &amp; Freddie&#39;s Diner&#34; &lt;tasty@example.com&gt;
  18. // &#34;Fran &amp; Freddie&#39;s Diner&#34; &lt;tasty@example.com&gt;
  19. // &#34;Fran &amp; Freddie&#39;s Diner&#34;32&lt;tasty@example.com&gt;
  20. // \"Fran & Freddie\'s Diner\" \x3Ctasty@example.com\x3E
  21. // \"Fran & Freddie\'s Diner\" \x3Ctasty@example.com\x3E
  22. // \"Fran & Freddie\'s Diner\"32\x3Ctasty@example.com\x3E
  23. // %22Fran+%26+Freddie%27s+Diner%2232%3Ctasty%40example.com%3E

Index

Package files

content.go css.go error.go html.go template.go url.go

func 

  1. func HTMLEscape(w .Writer, b [])

HTMLEscape writes to w the escaped HTML equivalent of the plain text data b.

func HTMLEscapeString

  1. func HTMLEscapeString(s string)

HTMLEscapeString returns the escaped HTML equivalent of the plain text data s.

func HTMLEscaper

  1. func HTMLEscaper(args ...interface{}) string

HTMLEscaper returns the escaped HTML equivalent of the textual representation of
its arguments.

func

IsTrue reports whether the value is ‘true’, in the sense of not the zero of its
type, and whether the value has a meaningful truth value. This is the definition
of truth used by if and other such actions.

  1. func JSEscape(w .Writer, b [])

JSEscape writes to w the escaped JavaScript equivalent of the plain text data b.

func JSEscapeString

  1. func JSEscapeString(s string)

JSEscapeString returns the escaped JavaScript equivalent of the plain text data
s.

func JSEscaper

    JSEscaper returns the escaped JavaScript equivalent of the textual
    representation of its arguments.

    func URLQueryEscaper

    1. func URLQueryEscaper(args ...interface{}) string

    URLQueryEscaper returns the escaped value of the textual representation of its
    arguments in a form suitable for embedding in a URL query.

    type 

    1. type CSS

    CSS encapsulates known safe content that matches any of:

    1. 1. The CSS3 stylesheet production, such as `p { color: purple }`.
    2. 2. The CSS3 rule production, such as `a[href=~"https:"].foo#bar`.
    3. 3. CSS3 declaration productions, such as `color: red; margin: 2px`.
    4. 4. The CSS3 value production, such as `rgba(0, 0, 255, 127)`.

    See http://www.w3.org/TR/css3-syntax/#parsing and

    Use of this type presents a security risk: the encapsulated content should come
    from a trusted source, as it will be included verbatim in the template output.

    type Error

    1. type Error struct {
    2.     // ErrorCode describes the kind of error.
    3.     ErrorCode ErrorCode
    4.     // Node is the node that caused the problem, if known.
    5.     // If not nil, it overrides Name and Line.
    6.     Node .Node
    7.     // Name is the name of the template in which the error was encountered.
    8.     Name 
    9.     // Line is the line number of the error in the template source or 0.
    10.     Line int
    11.     // Description is a human-readable description of the problem.
    12.     Description 
    13. }

    Error describes a problem encountered during template Escaping.

    func (*Error) Error

    1. func (e *Error) Error()

    type ErrorCode

    1. type ErrorCode int

    ErrorCode is a code for a kind of error.

    1. const (
    2.     // OK indicates the lack of an error.
    3.     OK  = iota
    4.  
    5.     // ErrAmbigContext: "... appears in an ambiguous context within a URL"
    6.     // Example:
    7.     //   <a href="
    8.     //      {{if .C}}
    9.     //        /path/
    10.     //      {{else}}
    11.     //        /search?q=
    12.     //      {{end}}
    13.     //      {{.X}}
    14.     //   ">
    15.     // Discussion:
    16.     //   {{.X}} is in an ambiguous URL context since, depending on {{.C}},
    17.     //  it may be either a URL suffix or a query parameter.
    18.     //   Moving {{.X}} into the condition removes the ambiguity:
    19.     //   <a href="{{if .C}}/path/{{.X}}{{else}}/search?q={{.X}}">
    20.     ErrAmbigContext
    21.  
    22.     // ErrBadHTML: "expected space, attr name, or end of tag, but got ...",
    23.     //   "... in unquoted attr", "... in attribute name"
    24.     // Example:
    25.     //   <a href = /search?q=foo>
    26.     //   <href=foo>
    27.     //   <form na<e=...>
    28.     //   <option selected<
    29.     // Discussion:
    30.     //   This is often due to a typo in an HTML element, but some runes
    31.     //   are banned in tag names, attribute names, and unquoted attribute
    32.     //   values because they can tickle parser ambiguities.
    33.     //   Quoting all attributes is the best policy.
    34.     ErrBadHTML
    35.  
    36.     // ErrBranchEnd: "{{if}} branches end in different contexts"
    37.     // Example:
    38.     //   {{if .C}}<a href="{{end}}{{.X}}
    39.     // Discussion:
    40.     //   Package html/template statically examines each path through an
    41.     //   {{if}}, {{range}}, or {{with}} to escape any following pipelines.
    42.     //   The example is ambiguous since {{.X}} might be an HTML text node,
    43.     //   or a URL prefix in an HTML attribute. The context of {{.X}} is
    44.     //   used to figure out how to escape it, but that context depends on
    45.     //   the run-time value of {{.C}} which is not statically known.
    46.     //
    47.     //   The problem is usually something like missing quotes or angle
    48.     //   brackets, or can be avoided by refactoring to put the two contexts
    49.     //   into different branches of an if, range or with. If the problem
    50.     //   is in a {{range}} over a collection that should never be empty,
    51.     //   adding a dummy {{else}} can help.
    52.     ErrBranchEnd
    53.  
    54.     // ErrEndContext: "... ends in a non-text context: ..."
    55.     // Examples:
    56.     //   <div
    57.     //   <div title="no close quote>
    58.     //   <script>f()
    59.     // Discussion:
    60.     //   Executed templates should produce a DocumentFragment of HTML.
    61.     //   Templates that end without closing tags will trigger this error.
    62.     //   Templates that should not be used in an HTML context or that
    63.     //   produce incomplete Fragments should not be executed directly.
    64.     //
    65.     //   {{define "main"}} <script>{{template "helper"}}</script> {{end}}
    66.     //   {{define "helper"}} document.write(' <div title=" ') {{end}}
    67.     //
    68.     //   "helper" does not produce a valid document fragment, so should
    69.     //   not be Executed directly.
    70.     ErrEndContext
    71.  
    72.     // ErrNoSuchTemplate: "no such template ..."
    73.     // Examples:
    74.     //   {{define "main"}}<div {{template "attrs"}}>{{end}}
    75.     //   {{define "attrs"}}href="{{.URL}}"{{end}}
    76.     // Discussion:
    77.     //   Package html/template looks through template calls to compute the
    78.     //   context.
    79.     //   Here the {{.URL}} in "attrs" must be treated as a URL when called
    80.     //   from "main", but you will get this error if "attrs" is not defined
    81.     //   when "main" is parsed.
    82.     ErrNoSuchTemplate
    83.  
    84.     // ErrOutputContext: "cannot compute output context for template ..."
    85.     // Examples:
    86.     //   {{define "t"}}{{if .T}}{{template "t" .T}}{{end}}{{.H}}",{{end}}
    87.     // Discussion:
    88.     //   A recursive template does not end in the same context in which it
    89.     //   starts, and a reliable output context cannot be computed.
    90.     //   Look for typos in the named template.
    91.     //   If the template should not be called in the named start context,
    92.     //   look for calls to that template in unexpected contexts.
    93.     //   Maybe refactor recursive templates to not be recursive.
    94.     ErrOutputContext
    95.  
    96.     // ErrPartialCharset: "unfinished JS regexp charset in ..."
    97.     // Example:
    98.     //     <script>var pattern = /foo[{{.Chars}}]/</script>
    99.     // Discussion:
    100.     //   Package html/template does not support interpolation into regular
    101.     //   expression literal character sets.
    102.     ErrPartialCharset
    103.  
    104.     // ErrPartialEscape: "unfinished escape sequence in ..."
    105.     // Example:
    106.     //   <script>alert("\{{.X}}")</script>
    107.     // Discussion:
    108.     //   Package html/template does not support actions following a
    109.     //   backslash.
    110.     //   This is usually an error and there are better solutions; for
    111.     //   example
    112.     //     <script>alert("{{.X}}")</script>
    113.     //   should work, and if {{.X}} is a partial escape sequence such as
    114.     //   "xA0", mark the whole sequence as safe content: JSStr(`\xA0`)
    115.     ErrPartialEscape
    116.  
    117.     // ErrRangeLoopReentry: "on range loop re-entry: ..."
    118.     // Example:
    119.     //   <script>var x = [{{range .}}'{{.}},{{end}}]</script>
    120.     // Discussion:
    121.     //   If an iteration through a range would cause it to end in a
    122.     //   different context than an earlier pass, there is no single context.
    123.     //   In the example, there is missing a quote, so it is not clear
    124.     //   whether {{.}} is meant to be inside a JS string or in a JS value
    125.     //   context. The second iteration would produce something like
    126.     //
    127.     //     <script>var x = ['firstValue,'secondValue]</script>
    128.     ErrRangeLoopReentry
    129.  
    130.     // ErrSlashAmbig: '/' could start a division or regexp.
    131.     // Example:
    132.     //   <script>
    133.     //     {{if .C}}var x = 1{{end}}
    134.     //     /-{{.N}}/i.test(x) ? doThis : doThat();
    135.     //   </script>
    136.     // Discussion:
    137.     //   The example above could produce `var x = 1/-2/i.test(s)...`
    138.     //   in which the first '/' is a mathematical division operator or it
    139.     //   could produce `/-2/i.test(s)` in which the first '/' starts a
    140.     //   regexp literal.
    141.     //   Look for missing semicolons inside branches, and maybe add
    142.     //   parentheses to make it clear which interpretation you intend.
    143.     ErrSlashAmbig
    144.  
    145.     // ErrPredefinedEscaper: "predefined escaper ... disallowed in template"
    146.     // Example:
    147.     //   <div class={{. | html}}>Hello<div>
    148.     // Discussion:
    149.     //   Package html/template already contextually escapes all pipelines to
    150.     //   produce HTML output safe against code injection. Manually escaping
    151.     //   pipeline output using the predefined escapers "html" or "urlquery" is
    152.     //   unnecessary, and may affect the correctness or safety of the escaped
    153.     //   pipeline output in Go 1.8 and earlier.
    154.     //
    155.     //   In most cases, such as the given example, this error can be resolved by
    156.     //   simply removing the predefined escaper from the pipeline and letting the
    157.     //   contextual autoescaper handle the escaping of the pipeline. In other
    158.     //   instances, where the predefined escaper occurs in the middle of a
    159.     //   pipeline where subsequent commands expect escaped input, e.g.
    160.     //     {{.X | html | makeALink}}
    161.     //   where makeALink does
    162.     //     return `<a href="`+input+`">link</a>`
    163.     //   consider refactoring the surrounding template to make use of the
    164.     //   contextual autoescaper, i.e.
    165.     //     <a href="{{.X}}">link</a>
    166.     //
    167.     //   To ease migration to Go 1.9 and beyond, "html" and "urlquery" will
    168.     //   continue to be allowed as the last command in a pipeline. However, if the
    169.     //   pipeline occurs in an unquoted attribute value context, "html" is
    170.     //   disallowed. Avoid using "html" and "urlquery" entirely in new templates.
    171.     ErrPredefinedEscaper
    172. )

    We define codes for each error that manifests while escaping templates, but
    escaped templates may also fail at runtime.

    Output: “ZgotmplZ” Example:

    1. <img src="{{.X}}">
    2. where {{.X}} evaluates to `javascript:...`

    Discussion:

    1. "ZgotmplZ" is a special value that indicates that unsafe content reached a
    2.   <img src="#ZgotmplZ">
    3. If the data comes from a trusted source, use content types to exempt it
    4. from filtering: URL(`javascript:...`).

    1. type FuncMap map[]interface{}

    FuncMap is the type of the map defining the mapping from names to functions.
    Each function must have either a single return value, or two return values of
    which the second has type error. In that case, if the second (error) argument
    evaluates to non-nil during execution, execution terminates and Execute returns
    that error. FuncMap has the same base type as FuncMap in “text/template”, copied
    here so clients need not import “text/template”.

    type HTML

    1. type HTML string

    HTML encapsulates a known safe HTML document fragment. It should not be used for
    HTML from a third-party, or HTML with unclosed tags or comments. The outputs of
    a sound HTML sanitizer and a template escaped by this package are fine for use
    with HTML.

    Use of this type presents a security risk: the encapsulated content should come
    from a trusted source, as it will be included verbatim in the template output.

    type 

    1. type HTMLAttr

    HTMLAttr encapsulates an HTML attribute from a trusted source, for example, dir="ltr".

    Use of this type presents a security risk: the encapsulated content should come
    from a trusted source, as it will be included verbatim in the template output.

    type JS

    1. type JS string

    JS encapsulates a known safe EcmaScript5 Expression, for example, (x + y * z()). Template authors are responsible for ensuring that typed expressions do
    not break the intended precedence and that there is no statement/expression
    ambiguity as when passing an expression like “{ foo: bar() }\n“, which
    is both a valid Expression and a valid Program with a very different meaning.

    Use of this type presents a security risk: the encapsulated content should come
    from a trusted source, as it will be included verbatim in the template output.

    Using JS to include valid but untrusted JSON is not safe. A safe alternative is
    to parse the JSON with json.Unmarshal and then pass the resultant object into
    the template, where it will be converted to sanitized JSON when presented in a
    JavaScript context.

    type JSStr

    1. type JSStr string

    JSStr encapsulates a sequence of characters meant to be embedded between quotes
    in a JavaScript expression. The string must match a series of StringCharacters:

    1. StringCharacter :: SourceCharacter but not `\` or LineTerminator
    2.                  | EscapeSequence

    Note that LineContinuations are not allowed. JSStr(“foo\nbar”) is fine, but
    JSStr(“foo\\nbar”) is not.

    Use of this type presents a security risk: the encapsulated content should come
    from a trusted source, as it will be included verbatim in the template output.

    type 

    1. type Srcset

    Srcset encapsulates a known safe srcset attribute (see
    http://w3c.github.io/html/semantics-embedded-content.html#element-attrdef-img-srcset).

    Use of this type presents a security risk: the encapsulated content should come
    from a trusted source, as it will be included verbatim in the template output.

    type 

    1. type Template struct {
    2.  
    3.     // The underlying template's parse tree, updated to be HTML-safe.
    4.     Tree *.Tree
    5.     // contains filtered or unexported fields
    6. }

    Template is a specialized Template from “text/template” that produces a safe
    HTML document fragment.


    Example:

    1. const (
    2.     master  = `Names:{{block "list" .}}{{"\n"}}{{range .}}{{println "-" .}}{{end}}{{end}}`
    3.     overlay = `{{define "list"}} {{join . ", "}}{{end}} `
    4. )
    5. var (
    6.     funcs     = template.FuncMap{"join": strings.Join}
    7.     guardians = []string{"Gamora", "Groot", "Nebula", "Rocket", "Star-Lord"}
    8. )
    9. masterTmpl, err := template.New("master").Funcs(funcs).Parse(master)
    10. if err != nil {
    11.     log.Fatal(err)
    12. }
    13. overlayTmpl, err := template.Must(masterTmpl.Clone()).Parse(overlay)
    14. if err != nil {
    15.     log.Fatal(err)
    16. }
    17. if err := masterTmpl.Execute(os.Stdout, guardians); err != nil {
    18.     log.Fatal(err)
    19. }
    20. if err := overlayTmpl.Execute(os.Stdout, guardians); err != nil {
    21.     log.Fatal(err)
    22. }
    23. // Output:
    24. // Names:
    25. // - Gamora
    26. // - Groot
    27. // - Nebula
    28. // - Rocket
    29. // - Star-Lord
    30. // Names: Gamora, Groot, Nebula, Rocket, Star-Lord


    Example:

    1. // Here we create a temporary directory and populate it with our sample
    2. // template definition files; usually the template files would already
    3. // exist in some location known to the program.
    4. dir := createTestDir([]templateFile{
    5.     // T0.tmpl is a plain template file that just invokes T1.
    6.     {"T0.tmpl", `T0 invokes T1: ({{template "T1"}})`},
    7.     // T1.tmpl defines a template, T1 that invokes T2.
    8.     {"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
    9.     // T2.tmpl defines a template T2.
    10.     {"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
    11. })
    12. // Clean up after the test; another quirk of running as an example.
    13. defer os.RemoveAll(dir)
    15. // pattern is the glob pattern used to find all the template files.
    16. pattern := filepath.Join(dir, "*.tmpl")
    18. // Here starts the example proper.
    19. // T0.tmpl is the first name matched, so it becomes the starting template,
    20. // the value returned by ParseGlob.
    21. tmpl := template.Must(template.ParseGlob(pattern))
    23. err := tmpl.Execute(os.Stdout, nil)
    24. if err != nil {
    26. }
    27. // Output:
    28. // T0 invokes T1: (T1 invokes T2: (This is T2))


    Example:


    Example:

    1. // Here we create different temporary directories and populate them with our sample
    2. // template definition files; usually the template files would already
    3. // exist in some location known to the program.
    4. dir1 := createTestDir([]templateFile{
    5.     // T1.tmpl is a plain template file that just invokes T2.
    6.     {"T1.tmpl", `T1 invokes T2: ({{template "T2"}})`},
    7. })
    9. dir2 := createTestDir([]templateFile{
    10.     // T2.tmpl defines a template T2.
    11.     {"T2.tmpl", `{{define "T2"}}This is T2{{end}}`},
    12. })
    14. // Clean up after the test; another quirk of running as an example.
    15. defer func(dirs ...string) {
    16.     for _, dir := range dirs {
    17.         os.RemoveAll(dir)
    18.     }
    19. }(dir1, dir2)
    21. // Here starts the example proper.
    22. // Let's just parse only dir1/T0 and dir2/T2
    23. paths := []string{
    24.     filepath.Join(dir1, "T1.tmpl"),
    25.     filepath.Join(dir2, "T2.tmpl"),
    26. }
    27. tmpl := template.Must(template.ParseFiles(paths...))
    29. err := tmpl.Execute(os.Stdout, nil)
    30. if err != nil {
    31.     log.Fatalf("template execution: %s", err)
    32. }
    33. // Output:
    34. // T1 invokes T2: (This is T2)


    Example:

    1. // Here we create a temporary directory and populate it with our sample
    2. // template definition files; usually the template files would already
    3. // exist in some location known to the program.
    4. dir := createTestDir([]templateFile{
    5.     // T0.tmpl is a plain template file that just invokes T1.
    6.     {"T0.tmpl", "T0 ({{.}} version) invokes T1: ({{template `T1`}})\n"},
    7.     // T1.tmpl defines a template, T1 that invokes T2. Note T2 is not defined
    8.     {"T1.tmpl", `{{define "T1"}}T1 invokes T2: ({{template "T2"}}){{end}}`},
    9. })
    10. // Clean up after the test; another quirk of running as an example.
    11. defer os.RemoveAll(dir)
    13. // pattern is the glob pattern used to find all the template files.
    14. pattern := filepath.Join(dir, "*.tmpl")
    16. // Here starts the example proper.
    17. // Load the drivers.
    18. drivers := template.Must(template.ParseGlob(pattern))
    20. // We must define an implementation of the T2 template. First we clone
    21. // the drivers, then add a definition of T2 to the template name space.
    23. // 1. Clone the helper set to create a new name space from which to run them.
    24. first, err := drivers.Clone()
    25. if err != nil {
    26.     log.Fatal("cloning helpers: ", err)
    27. }
    28. // 2. Define T2, version A, and parse it.
    29. _, err = first.Parse("{{define `T2`}}T2, version A{{end}}")
    30. if err != nil {
    31.     log.Fatal("parsing T2: ", err)
    32. }
    34. // Now repeat the whole thing, using a different version of T2.
    35. // 1. Clone the drivers.
    36. second, err := drivers.Clone()
    37. if err != nil {
    38.     log.Fatal("cloning drivers: ", err)
    39. }
    40. // 2. Define T2, version B, and parse it.
    41. _, err = second.Parse("{{define `T2`}}T2, version B{{end}}")
    42. if err != nil {
    43.     log.Fatal("parsing T2: ", err)
    44. }
    46. // Execute the templates in the reverse order to verify the
    47. // first is unaffected by the second.
    48. err = second.ExecuteTemplate(os.Stdout, "T0.tmpl", "second")
    49. if err != nil {
    50.     log.Fatalf("second execution: %s", err)
    51. }
    52. err = first.ExecuteTemplate(os.Stdout, "T0.tmpl", "first")
    53. if err != nil {
    54.     log.Fatalf("first: execution: %s", err)
    55. }
    57. // Output:
    58. // T0 (second version) invokes T1: (T1 invokes T2: (T2, version B))
    59. // T0 (first version) invokes T1: (T1 invokes T2: (T2, version A))

    func Must

    1. func Must(t *Template, err ) *Template

    Must is a helper that wraps a call to a function returning (*Template, error)
    and panics if the error is non-nil. It is intended for use in variable
    initializations such as

    1. var t = template.Must(template.New("name").Parse("html"))

    func 

    1. func New(name ) *Template

    New allocates a new HTML template with the given name.

    func 

    1. func ParseFiles(filenames ...) (*Template, )

    When parsing multiple files with the same name in different directories, the
    last one mentioned will be the one that results. For instance,
    ParseFiles(“a/foo”, “b/foo”) stores “b/foo” as the template named “foo”, while
    “a/foo” is unavailable.

    func ParseGlob

    1. func ParseGlob(pattern string) (*, error)

    ParseGlob creates a new Template and parses the template definitions from the
    files identified by the pattern, which must match at least one file. The
    returned template will have the (base) name and (parsed) contents of the first
    file matched by the pattern. ParseGlob is equivalent to calling ParseFiles with
    the list of files matched by the pattern.

    When parsing multiple files with the same name in different directories, the
    last one mentioned will be the one that results.

    1. func (t *) AddParseTree(name string, tree *.Tree) (*, error)

    AddParseTree creates a new template with the name and parse tree and associates
    it with t.

    It returns an error if t or any associated template has already been executed.

    func (*Template) 

    1. func (t *) Clone() (*Template, )

    Clone returns a duplicate of the template, including all associated templates.
    The actual representation is not copied, but the name space of associated
    templates is, so further calls to Parse in the copy will add templates to the
    copy but not to the original. Clone can be used to prepare common templates and
    use them with variant definitions for other templates by adding the variants
    after the clone is made.

    It returns an error if t has already been executed.

    func (*Template) DefinedTemplates

    1. func (t *Template) DefinedTemplates()

    DefinedTemplates returns a string listing the defined templates, prefixed by the
    string “; defined templates are: “. If there are none, it returns the empty
    string. Used to generate an error message.

    func (*Template) Delims

    1. func (t *Template) Delims(left, right ) *Template

    Delims sets the action delimiters to the specified strings, to be used in
    subsequent calls to Parse, ParseFiles, or ParseGlob. Nested template definitions
    will inherit the settings. An empty delimiter stands for the corresponding
    default: {{ or }}. The return value is the template, so calls can be chained.

    func (*Template) 

    1. func (t *) Execute(wr io., data interface{}) error

    Execute applies a parsed template to the specified data object, writing the
    output to wr. If an error occurs executing the template or writing its output,
    execution stops, but partial results may already have been written to the output
    writer. A template may be executed safely in parallel, although if parallel
    executions share a Writer the output may be interleaved.

    func (*Template) 

    1. func (t *) ExecuteTemplate(wr io., name string, data interface{})

    ExecuteTemplate applies the template associated with t that has the given name
    to the specified data object and writes the output to wr. If an error occurs
    executing the template or writing its output, execution stops, but partial
    results may already have been written to the output writer. A template may be
    executed safely in parallel, although if parallel executions share a Writer the
    output may be interleaved.

    func (*Template) Funcs

    1. func (t *Template) Funcs(funcMap ) *Template

    Funcs adds the elements of the argument map to the template’s function map. It
    must be called before the template is parsed. It panics if a value in the map is
    not a function with appropriate return type. However, it is legal to overwrite
    elements of the map. The return value is the template, so calls can be chained.

    1. func (t *) Lookup(name string) *

    Lookup returns the template with the given name that is associated with t, or
    nil if there is no such template.

    func (*Template) Name

    1. func (t *Template) Name()

    Name returns the name of the template.

    func (*Template) New

    1. func (t *Template) New(name ) *Template

    New allocates a new HTML template associated with the given one and with the
    same delimiters. The association, which is transitive, allows one template to
    invoke another with a {{template}} action.

    If a template with the given name already exists, the new HTML template will
    replace it. The existing template will be reset and disassociated with t.

    func (*Template) 

    1. func (t *) Option(opt ...string) *

    Option sets options for the template. Options are described by strings, either a
    simple string or “key=value”. There can be at most one equals sign in an option
    string. If the option string is unrecognized or otherwise invalid, Option
    panics.

    Known options:

    missingkey: Control the behavior during execution if a map is indexed with a key
    that is not present in the map.

    1. "missingkey=default" or "missingkey=invalid"
    2.     The default behavior: Do nothing and continue execution.
    3.     If printed, the result of the index operation is the string
    4.     "<no value>".
    5. "missingkey=zero"
    6.     The operation returns the zero value for the map type's element.
    7. "missingkey=error"
    8.     Execution stops immediately with an error.

    func (*Template) Parse

    1. func (t *Template) Parse(text ) (*Template, )

    Parse parses text as a template body for t. Named template definitions ({{define
    …}} or {{block …}} statements) in text define additional templates
    associated with t and are removed from the definition of t itself.

    Templates can be redefined in successive calls to Parse, before the first use of
    Execute on t or any associated template. A template definition with a body
    containing only white space and comments is considered empty and will not
    replace an existing template’s body. This allows using Parse to add new named
    template definitions without overwriting the main template body.

    func (*Template) ParseFiles

    1. func (t *Template) ParseFiles(filenames ...) (*Template, )

    ParseFiles parses the named files and associates the resulting templates with t.
    If an error occurs, parsing stops and the returned template is nil; otherwise it
    is t. There must be at least one file.

    When parsing multiple files with the same name in different directories, the
    last one mentioned will be the one that results.

    ParseFiles returns an error if t or any associated template has already been
    executed.

    func (*Template) ParseGlob

    1. func (t *Template) ParseGlob(pattern ) (*Template, )

    ParseGlob parses the template definitions in the files identified by the pattern
    and associates the resulting templates with t. The pattern is processed by
    filepath.Glob and must match at least one file. ParseGlob is equivalent to
    calling t.ParseFiles with the list of files matched by the pattern.

    When parsing multiple files with the same name in different directories, the
    last one mentioned will be the one that results.

    ParseGlob returns an error if t or any associated template has already been
    executed.

    Templates returns a slice of the templates associated with t, including t
    itself.

    1. type URL string

    URL encapsulates a known safe URL or URL substring (see RFC 3986). A URL like
    from a trusted source
    should go in the page, but by default dynamic javascript: URLs are filtered
    out since they are a frequently exploited injection vector.

    Use of this type presents a security risk: the encapsulated content should come
    from a trusted source, as it will be included verbatim in the template output.