Binder Container

An AST node can be a container. This determines the kinds of SymbolTables the Node and associated Symbol will have. Container is an abstract concept (i.e. has no associated data structure). The concept is driven by a few things, one being the ContainerFlags enum. The function getContainerFlags (in binder.ts) drives this flag and is presented below:

function getContainerFlags(node: Node): ContainerFlags {
switch (node.kind) {
case SyntaxKind.ClassExpression:
case SyntaxKind.ClassDeclaration:
case SyntaxKind.InterfaceDeclaration:
case SyntaxKind.EnumDeclaration:
case SyntaxKind.TypeLiteral:
case SyntaxKind.ObjectLiteralExpression:
return ContainerFlags.IsContainer;
case SyntaxKind.CallSignature:
case SyntaxKind.ConstructSignature:
case SyntaxKind.IndexSignature:
case SyntaxKind.MethodDeclaration:
case SyntaxKind.MethodSignature:
case SyntaxKind.FunctionDeclaration:
case SyntaxKind.Constructor:
case SyntaxKind.GetAccessor:
case SyntaxKind.SetAccessor:
case SyntaxKind.FunctionType:
case SyntaxKind.ConstructorType:
case SyntaxKind.FunctionExpression:
case SyntaxKind.ArrowFunction:
case SyntaxKind.ModuleDeclaration:
case SyntaxKind.SourceFile:
case SyntaxKind.TypeAliasDeclaration:
return ContainerFlags.IsContainerWithLocals;
case SyntaxKind.CatchClause:
case SyntaxKind.ForStatement:
case SyntaxKind.ForInStatement:
case SyntaxKind.ForOfStatement:
case SyntaxKind.CaseBlock:
return ContainerFlags.IsBlockScopedContainer;
case SyntaxKind.Block:
// do not treat blocks directly inside a function as a block-scoped-container.
// Locals that reside in this block should go to the function locals. Otherwise 'x'
// would not appear to be a redeclaration of a block scoped local in the following
// example:
// function foo() {
// var x;
// let x;
// }
// If we placed 'var x' into the function locals and 'let x' into the locals of
// the block, then there would be no collision.
// By not creating a new block-scoped-container here, we ensure that both 'var x'
// and 'let x' go into the Function-container's locals, and we do get a collision
// conflict.
return isFunctionLike(node.parent) ? ContainerFlags.None : ContainerFlags.IsBlockScopedContainer;
return ContainerFlags.None;

It is only invoked from the binder's bindChildren function which sets up a node as a container and/or a blockScopedContainer depending upon the evaluation of the getContainerFlags function. The function bindChildren is presented below:

// All container nodes are kept on a linked list in declaration order. This list is used by
// the getLocalNameOfContainer function in the type checker to validate that the local name
// used for a container is unique.
function bindChildren(node: Node) {
// Before we recurse into a node's children, we first save the existing parent, container
// and block-container. Then after we pop out of processing the children, we restore
// these saved values.
let saveParent = parent;
let saveContainer = container;
let savedBlockScopeContainer = blockScopeContainer;
// This node will now be set as the parent of all of its children as we recurse into them.
parent = node;
// Depending on what kind of node this is, we may have to adjust the current container
// and block-container. If the current node is a container, then it is automatically
// considered the current block-container as well. Also, for containers that we know
// may contain locals, we proactively initialize the .locals field. We do this because
// it's highly likely that the .locals will be needed to place some child in (for example,
// a parameter, or variable declaration).
// However, we do not proactively create the .locals for block-containers because it's
// totally normal and common for block-containers to never actually have a block-scoped
// variable in them. We don't want to end up allocating an object for every 'block' we
// run into when most of them won't be necessary.
// Finally, if this is a block-container, then we clear out any existing .locals object
// it may contain within it. This happens in incremental scenarios. Because we can be
// reusing a node from a previous compilation, that node may have had 'locals' created
// for it. We must clear this so we don't accidentally move any stale data forward from
// a previous compilation.
let containerFlags = getContainerFlags(node);
if (containerFlags & ContainerFlags.IsContainer) {
container = blockScopeContainer = node;
if (containerFlags & ContainerFlags.HasLocals) {
container.locals = {};
else if (containerFlags & ContainerFlags.IsBlockScopedContainer) {
blockScopeContainer = node;
blockScopeContainer.locals = undefined;
forEachChild(node, bind);
container = saveContainer;
parent = saveParent;
blockScopeContainer = savedBlockScopeContainer;

As you might recall from the section on binder functions : bindChildren is called from the bind function. So we have the recursive binding setup : bind calls bindChildren calls bind for each child.