cst

JavaScript CST implementation

CST

CST means Concrete Syntax Tree. Unlike an AST (Abstract Syntax Tree), a CST contains all the information from the JavaScript source file: whitespace, punctuators, comments. This information is extremely useful for code style checkers and other code linters. CST is also useful for cases when you need to apply modifications to existing JavaScript files while preserving the initial file formatting.

This CST implementation is designed to be 100% compatible with JS AST (https://github.com/estree/estree).

Main principles:

• CST contains all the information from a parsed file (including whitespace and comments).
• Compatible with AST (https://github.com/estree/estree).
• Requires tokens to modify CST structure.
• The tree is always valid (it protects itself against breaking changes).
• CST can be rendered to valid JS at any time.

Let's see an example:

x = 0;
if (x) x++;

The CST for this example:

• Blue text — CST Tokens.
• White text in blue blocks — CST Nodes (their structure is equal to an AST).
• Blue lines — CST Structure.
• Red lined — AST Links.

Classes

Element

Element is the base class for Node and Token.

Provides traversing properties:

• childElements: Element[], parentElement: Element|null: child/parent traversing.
• nextSibling: Element|null, previousSibling: Element|null: traversing between siblings.
• nextToken: Token|null, previousToken: Token|null: traversing to next/previous token.
• firstToken: Token|null, lastToken: Token|null: traversing to first/last tokens (not only direct tokens).
• firstChild: Token|null, lastChild: Token|null: traversing to first/last direct child.

Code-related properties:

• sourceCode: generates and returns JavaScript code of the specified Element
• sourceCodeLength: returns JavaScript code length
• isToken, isNode, isExpression, isStatement, isWhitespace, isComment, isPattern, isAssignable, isFragment, isModuleDeclaration, isModuleSpecifier : code entity flags.

Provides mutation methods:

• appendChild(element): appends child to the end of the Element
• prependChild(element): prepends child to the end of the Element
• insertChildBefore(element, referenceChild): inserts child before referenceChild
• replaceChildren(element, firstChildRef, lastChildRef): replaces specified child interval (from firstChildRef to lastChildRef) with specified child.

Location properties:

• range: [Number, Number]: calculates and returns Element range.
• loc: {start: {line: Number, column: Number}, end: {line: Number, column: Number}}: calculates and returns Element location.

Node

Node extends Element. The Nodes are the "AST part of a CST". If you drop everything but Nodes from a CST, you will get a pure AST from the Node structure. So it is fair to say that Nodes provide the AST logic for a CST. Currently only Nodes can contain children.

The Node property isNode always returns true.

Token

Token extends Element. The purpose of a CST is to have tokens in the tree. By only manipulating tokens, we can change code formatting without any effect on the behaviour.

The Token property isToken always returns true.