AST Tip: Visit Children

There is a utility function `ts.forEachChild` that allows you to visit all the child nodes of any Node in the AST.

Here is simplified snippet of the source code to demonstrate how it functions:

``````
export function forEachChild<T>(node: Node, cbNode: (node: Node) => T, cbNodeArray?: (nodes: Node[]) => T): T {
if (!node) {
return;
}
switch (node.kind) {
case SyntaxKind.BinaryExpression:
return visitNode(cbNode, (<BinaryExpression>node).left) ||
visitNode(cbNode, (<BinaryExpression>node).operatorToken) ||
visitNode(cbNode, (<BinaryExpression>node).right);
case SyntaxKind.IfStatement:
return visitNode(cbNode, (<IfStatement>node).expression) ||
visitNode(cbNode, (<IfStatement>node).thenStatement) ||
visitNode(cbNode, (<IfStatement>node).elseStatement);

// .... lots more
``````

Basically, it checks `node.kind` and based on that assumes an interface offered by the `node` and calls the `cbNode` on the children. However, note that this function doesn't call `visitNode` for all children (e.g. SyntaxKind.SemicolonToken). If you want all the children of a node in the AST just call `.getChildren` member function of the `Node`.

E.g. here is a function that prints the verbose `AST` of a node:

``````function printAllChildren(node: ts.Node, depth = 0) {
console.log(new Array(depth+1).join('----'), ts.syntaxKindToName(node.kind), node.pos, node.end);
depth++;
node.getChildren().forEach(c=> printAllChildren(c, depth));
}
``````

We will see a sample usage of this function when we discuss the parser further.