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# Handlebars Compiler APIs
There are a number of formal APIs that tool implementors may interact with.
## AST
Other tools may interact with the formal AST as defined below. Any JSON structure matching this pattern may be used and passed into the `compile` and `precompile` methods in the same way as the text for a template.
AST structures may be generated either with the `Handlebars.parse` method and then manipulated, via the `Handlebars.AST` objects of the same name, or constructed manually as a generic JavaScript object matching the structure defined below.
```javascript
var ast = Handlebars.parse(myTemplate);
// Modify ast
Handlebars.precompile(ast);
```
### Basic
```java
interface Node {
type: string;
loc: SourceLocation | null;
}
interface SourceLocation {
source: string | null;
start: Position;
end: Position;
}
interface Position {
line: uint >= 1;
column: uint >= 0;
}
```
### Programs
```java
interface Program <: Node {
type: "Program";
body: [ Statement ];
blockParams: [ string ];
}
```
### Statements
```java
interface Statement <: Node { }
interface MustacheStatement <: Statement {
type: "MustacheStatement";
path: PathExpression | Literal;
params: [ Expression ];
hash: Hash;
escaped: boolean;
strip: StripFlags | null;
}
interface BlockStatement <: Statement {
type: "BlockStatement";
path: PathExpression | Literal;
params: [ Expression ];
hash: Hash;
program: Program | null;
inverse: Program | null;
openStrip: StripFlags | null;
inverseStrip: StripFlags | null;
closeStrip: StripFlags | null;
}
interface PartialStatement <: Statement {
type: "PartialStatement";
name: PathExpression | SubExpression;
params: [ Expression ];
hash: Hash;
indent: string;
strip: StripFlags | null;
}
interface PartialBlockStatement <: Statement {
type: "PartialBlockStatement";
name: PathExpression | SubExpression;
params: [ Expression ];
hash: Hash;
program: Program | null;
indent: string;
openStrip: StripFlags | null;
closeStrip: StripFlags | null;
}
```
`name` will be a `SubExpression` when tied to a dynamic partial, i.e. `{{> (foo) }}`, otherwise this is a path or literal whose `original` value is used to lookup the desired partial.
```java
interface ContentStatement <: Statement {
type: "ContentStatement";
value: string;
original: string;
}
interface CommentStatement <: Statement {
type: "CommentStatement";
value: string;
strip: StripFlags | null;
}
```
```java
interface Decorator <: Statement {
type: "Decorator";
path: PathExpression | Literal;
params: [ Expression ];
hash: Hash;
strip: StripFlags | null;
}
interface DecoratorBlock <: Statement {
type: "DecoratorBlock";
path: PathExpression | Literal;
params: [ Expression ];
hash: Hash;
program: Program | null;
openStrip: StripFlags | null;
closeStrip: StripFlags | null;
}
```
Decorator paths only utilize the `path.original` value and as a consequence do not support depthed evaluation.
### Expressions
```java
interface Expression <: Node { }
```
##### SubExpressions
```java
interface SubExpression <: Expression {
type: "SubExpression";
path: PathExpression;
params: [ Expression ];
hash: Hash;
}
```
##### Paths
```java
interface PathExpression <: Expression {
type: "PathExpression";
data: boolean;
depth: uint >= 0;
parts: [ string ];
original: string;
}
```
- `data` is true when the given expression is a `@data` reference.
- `depth` is an integer representation of which context the expression references. `0` represents the current context, `1` would be `../`, etc.
- `parts` is an array of the names in the path. `foo.bar` would be `['foo', 'bar']`. Scope references, `.`, `..`, and `this` should be omitted from this array.
- `original` is the path as entered by the user. Separator and scope references are left untouched.
##### Literals
```java
interface Literal <: Expression { }
interface StringLiteral <: Literal {
type: "StringLiteral";
value: string;
original: string;
}
interface BooleanLiteral <: Literal {
type: "BooleanLiteral";
value: boolean;
original: boolean;
}
interface NumberLiteral <: Literal {
type: "NumberLiteral";
value: number;
original: number;
}
interface UndefinedLiteral <: Literal {
type: "UndefinedLiteral";
}
interface NullLiteral <: Literal {
type: "NullLiteral";
}
```
### Miscellaneous
```java
interface Hash <: Node {
type: "Hash";
pairs: [ HashPair ];
}
interface HashPair <: Node {
type: "HashPair";
key: string;
value: Expression;
}
interface StripFlags {
open: boolean;
close: boolean;
}
```
`StripFlags` are used to signify whitespace control character that may have been entered on a given statement.
## AST Visitor
`Handlebars.Visitor` is available as a base class for general interaction with AST structures. This will by default traverse the entire tree and individual methods may be overridden to provide specific responses to particular nodes.
Recording all referenced partial names:
```javascript
var Visitor = Handlebars.Visitor;
function ImportScanner() {
this.partials = [];
}
ImportScanner.prototype = new Visitor();
ImportScanner.prototype.PartialStatement = function(partial) {
this.partials.push({request: partial.name.original});
Visitor.prototype.PartialStatement.call(this, partial);
};
var scanner = new ImportScanner();
scanner.accept(ast);
```
The current node's ancestors will be maintained in the `parents` array, with the most recent parent listed first.
The visitor may also be configured to operate in mutation mode by setting the `mutation` field to true. When in this mode, handler methods may return any valid AST node and it will replace the one they are currently operating on. Returning `false` will remove the given value (if valid) and returning `undefined` will leave the node in tact. This return structure only apply to mutation mode and non-mutation mode visitors are free to return whatever values they wish.
Implementors that may need to support mutation mode are encouraged to utilize the `acceptKey`, `acceptRequired` and `acceptArray` helpers which provide the conditional overwrite behavior as well as implement sanity checks where pertinent.
## JavaScript Compiler
The `Handlebars.JavaScriptCompiler` object has a number of methods that may be customized to alter the output of the compiler:
- `nameLookup(parent, name, type)`
Used to generate the code to resolve a give path component.
- `parent` is the existing code in the path resolution
- `name` is the current path component
- `type` is the type of name being evaluated. May be one of `context`, `data`, `helper`, `decorator`, or `partial`.
Note that this does not impact dynamic partials, which implementors need to be aware of. Overriding `VM.resolvePartial` may be required to support dynamic cases.
- `depthedLookup(name)`
Used to generate code that resolves parameters within any context in the stack. Is only used in `compat` mode.
- `compilerInfo()`
Allows for custom compiler flags used in the runtime version checking logic.
- `appendToBuffer(source, location, explicit)`
Allows for code buffer emitting code. Defaults behavior is string concatenation.
- `source` is the source code whose result is to be appending
- `location` is the location of the source in the source map.
- `explicit` is a flag signaling that the emit operation must occur, vs. the lazy evaled options otherwise.
- `initializeBuffer()`
Allows for buffers other than the default string buffer to be used. Generally needs to be paired with a custom `appendToBuffer` implementation.
### Example for the compiler api.
This example changes all lookups of properties are performed by a helper (`lookupLowerCase`) which looks for `test` if `{{Test}}` occurs in the template. This is just to illustrate how compiler behavior can be change.
There is also [a jsfiddle with this code](https://jsfiddle.net/9D88g/162/) if you want to play around with it.
```javascript
function MyCompiler() {
Handlebars.JavaScriptCompiler.apply(this, arguments);
}
MyCompiler.prototype = new Handlebars.JavaScriptCompiler();
// Use this compile to compile BlockStatment-Blocks
MyCompiler.prototype.compiler = MyCompiler
MyCompiler.prototype.nameLookup = function(parent, name, type) {
if (type === 'context') {
return this.source.functionCall('helpers.lookupLowerCase', '', [parent, JSON.stringify(name)])
} else {
return Handlebars.JavaScriptCompiler.prototype.nameLookup.call(this, parent, name, type);
}
}
var env = Handlebars.create();
env.registerHelper('lookupLowerCase', function(parent, name) {
return parent[name.toLowerCase()]
})
env.JavaScriptCompiler = MyCompiler;
var template = env.compile('{{#each Test}} ({{Value}}) {{/each}}');
console.log(template({
test: [
{value: 'a'},
{value: 'b'},
{value: 'c'}
]
}));
```

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# Decorators
Decorators allow for blocks to be annotated with metadata or wrapped in functionality prior to execution of the block. This may be used to communicate with the containing helper or to setup a particular state in the system prior to running the block.
Decorators are registered through similar methods as helpers, `registerDecorators` and `unregisterDecorators`. These can then be referenced via the friendly name in the template using the `{{* decorator}}` and `{{#* decorator}}{/decorator}}` syntaxes. These syntaxes are derivatives of the normal mustache syntax and as such have all of the same argument and whitespace behaviors.
Decorators are executed when the block program is instantiated and are passed `(program, props, container, context, data, blockParams, depths)`.
- `program`: The block to wrap
- `props`: Object used to set metadata on the final function. Any values set on this object will be set on the function, regardless of if the original function is replaced or not. Metadata should be applied using this object as values applied to `program` may be masked by subsequent decorators that may wrap `program`.
- `container`: The current runtime container
- `context`: The current context. Since the decorator is run before the block that contains it, this is the parent context.
- `data`: The current `@data` values
- `blockParams`: The current block parameters stack
- `depths`: The current context stack
Decorators may set values on `props` or return a modified function that wraps `program` in particular behaviors. If the decorator returns nothing, then `program` is left unaltered.
The [inline partial](https://github.com/wycats/handlebars.js/blob/master/lib/handlebars/decorators/inline.js) implementation provides an example of decorators being used for both metadata and wrapping behaviors.