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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package template
import (
// Strings of content from a trusted source.
type (
// CSS encapsulates known safe content that matches any of:
// 1. The CSS3 stylesheet production, such as `p { color: purple }`.
// 2. The CSS3 rule production, such as `a[href=~"https:"].foo#bar`.
// 3. CSS3 declaration productions, such as `color: red; margin: 2px`.
// 4. The CSS3 value production, such as `rgba(0, 0, 255, 127)`.
// See and
CSS 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.
HTML string
// HTMLAttr encapsulates an HTML attribute from a trusted source,
// for example, ` dir="ltr"`.
HTMLAttr 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['foo']()", which is both a valid Expression and a
// valid Program with a very different meaning.
JS 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:
// StringCharacter :: SourceCharacter but not `\` or LineTerminator
// | EscapeSequence
// Note that LineContinuations are not allowed.
// JSStr("foo\\nbar") is fine, but JSStr("foo\\\nbar") is not.
JSStr string
// URL encapsulates a known safe URL or URL substring (see RFC 3986).
// A URL like `javascript:checkThatFormNotEditedBeforeLeavingPage()`
// 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.
URL string
type contentType uint8
const (
contentTypePlain contentType = iota
// contentTypeUnsafe is used in attr.go for values that affect how
// embedded content and network messages are formed, vetted,
// or interpreted; or which credentials network messages carry.
// indirect returns the value, after dereferencing as many times
// as necessary to reach the base type (or nil).
func indirect(a interface{}) interface{} {
if a == nil {
return nil
if t := reflect.TypeOf(a); t.Kind() != reflect.Ptr {
// Avoid creating a reflect.Value if it's not a pointer.
return a
v := reflect.ValueOf(a)
for v.Kind() == reflect.Ptr && !v.IsNil() {
v = v.Elem()
return v.Interface()
var (
errorType = reflect.TypeOf((*error)(nil)).Elem()
fmtStringerType = reflect.TypeOf((*fmt.Stringer)(nil)).Elem()
// indirectToStringerOrError returns the value, after dereferencing as many times
// as necessary to reach the base type (or nil) or an implementation of fmt.Stringer
// or error,
func indirectToStringerOrError(a interface{}) interface{} {
if a == nil {
return nil
v := reflect.ValueOf(a)
for !v.Type().Implements(fmtStringerType) && !v.Type().Implements(errorType) && v.Kind() == reflect.Ptr && !v.IsNil() {
v = v.Elem()
return v.Interface()
// stringify converts its arguments to a string and the type of the content.
// All pointers are dereferenced, as in the text/template package.
func stringify(args ...interface{}) (string, contentType) {
if len(args) == 1 {
switch s := indirect(args[0]).(type) {
case string:
return s, contentTypePlain
case CSS:
return string(s), contentTypeCSS
case HTML:
return string(s), contentTypeHTML
case HTMLAttr:
return string(s), contentTypeHTMLAttr
case JS:
return string(s), contentTypeJS
case JSStr:
return string(s), contentTypeJSStr
case URL:
return string(s), contentTypeURL
for i, arg := range args {
args[i] = indirectToStringerOrError(arg)
return fmt.Sprint(args...), contentTypePlain