handlebars.js/docs/compiler-api.md

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.

var ast = Handlebars.parse(myTemplate);

// Modify ast

Handlebars.precompile(ast);

Basic

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

interface Program <: Node {
    type: "Program";
    body: [ Statement ];
    
    blockParams: [ string ];
}

Statements

interface Statement <: Node { }

interface MustacheStatement <: Statement {
    type: "MustacheStatement";
    sexpr: SubExpression;
    escaped: boolean;

    strip: StripFlags | null;
}

interface BlockStatement <: Statement {
    type: "BlockStatement";
    sexpr: SubExpression;
    program: Program | null;
    inverse: Program | null;

    openStrip: StripFlags | null;
    inverseStrip: StripFlags | null;
    closeStrip: StripFlags | null;
}

interface PartialStatement <: Statement {
    type: "PartialStatement";
    sexpr: SubExpression;
    
    indent: string;
    strip: StripFlags | null;
}

interface ContentStatement <: Statement {
    type: "ContentStatement";
    value: string;
    original: string;
}

interface CommentStatement <: Statement {
    type: "CommentStatement";
    value: string;

    strip: StripFlags | null;
}

Expressions

interface Expression <: Node { }

SubExpressions

interface SubExpression <: Expression {
    type: "SubExpression";
    path: PathExpression;
    params: [ Expression ];
    hash: Hash;
    
    isHelper: true | null;
}

isHelper is not required and is used to disambiguate between cases such as {{foo}} and (foo), which have slightly different call behaviors.

Paths

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

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;
}

Miscellaneous

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:

var Visitor = Handlebars.Visitor;

function ImportScanner() {
  this.partials = [];
}
ImportScanner.prototype = new Visitor();

ImportScanner.prototype.PartialStatement = function(partial) {
  this.partials.push({request: partial.sexpr.original});

  Visitor.prototype.PartialStatement.call(this, partial);
};

var scanner = new ImportScanner();
scanner.accept(ast);

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, or partial.

• 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.

function MyCompiler() {
  Handlebars.JavaScriptCompiler.apply(this, arguments);
}
MyCompiler.prototype = Object.create(Handlebars.JavaScriptCompiler);

MyCompiler.nameLookup = function(parent, name, type) {
  if (type === 'partial') {
    return 'MyPartialList[' + JSON.stringify(name) ']';
  } else {
    return Handlebars.JavaScriptCompiler.prototype.nameLookup.call(this, parent, name, type);
  }
};

var env = Handlebars.create();
env.JavaScriptCompiler = MyCompiler;
env.compile('my template');