127 lines
7 KiB
Markdown
127 lines
7 KiB
Markdown
# babel-walk
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Lightweight AST traversal tools for [Babel] ASTs.
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[Babel] supplies the wonderful [babel-traverse] module for walking Babel ASTs. Problem is, babel-traverse is very heavyweight, as it is designed to supply utilities to make all sorts of AST transformations possible. For simple AST walking without transformation, babel-traverse brings a lot of overhead.
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This module loosely implements the API of Acorn parser's [walk module], which is a lightweight AST walker for the ESTree AST format.
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In my tests, babel-walk's ancestor walker (the most complex walker provided by this module) is about 8 times faster than babel-traverse, if the visitors are cached and the same AST is used for all runs. It is about 16 times faster if a fresh AST is used every run.
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[](https://github.com/pugjs/babel-walk/actions?query=workflow%3A%22Publish+Canary%22)
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[](https://rollingversions.com/pugjs/babel-walk)
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[](https://www.npmjs.com/package/babel-walk)
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[babel]: https://babeljs.io/
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[babel-traverse]: https://github.com/thejameskyle/babel-handbook/blob/master/translations/en/plugin-handbook.md#toc-babel-traverse
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[walk module]: https://github.com/ternjs/acorn#distwalkjs
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## Installation
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```sh
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$ npm install babel-walk
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```
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## API
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```js
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var walk = require('babel-walk');
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```
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### walk.simple(visitors)(node, state)
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Do a simple walk over the AST. `node` should be the AST node to walk, and `visitors` an object containing Babel [visitors]. Each visitor function will be called as `(node, state)`, where `node` is the AST node, and `state` is the same `state` passed to `walk.simple`.
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When `walk.simple` is called with a fresh set of visitors, it will first "explode" the visitors (e.g. expanding `Visitor(node, state) {}` to `Visitor() { enter(node, state) {} }`). This exploding process can take some time, so it is recommended to cache the result of calling `walk.simple(visitors)` and communicate state leveraging the `state` parameter.
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All [babel-types] aliases (e.g. `Expression`) work, but the union syntax (e.g. `'Identifier|AssignmentPattern'(node, state) {}`) does not.
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### walk.ancestor(visitors)(node, state)
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Do a simple walk over the AST, but memoizing the ancestors of the node and making them available to the visitors. `node` should be the AST node to walk, and `visitors` an object containing Babel [visitors]. Each visitor function will be called as `(node, state, ancestors)`, where `node` is the AST node, `state` is the same `state` passed to `walk.ancestor`, and `ancestors` is an array of ancestors to the node (with the outermost node being `[0]` and the current node being `[ancestors.length - 1]`). If `state` is not specified in the call to `walk.ancestor`, the `state` parameter will be set to `ancestors`.
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When `walk.ancestor` is called with a fresh set of visitors, it will first "explode" the visitors (e.g. expanding `Visitor(node, state) {}` to `Visitor() { enter(node, state) {} }`). This exploding process can take some time, so it is recommended to cache the result of calling `walk.ancestor(visitors)` and communicate state leveraging the `state` parameter.
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All [babel-types] aliases (e.g. `Expression`) work, but the union syntax (e.g. `'Identifier|AssignmentPattern'(node, state) {}`) does not.
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### walk.recursive(visitors)(node, state)
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Do a recursive walk over the AST, where the visitors are responsible for continuing the walk on the child nodes of their target node. `node` should be the AST node to walk, and `visitors` an object containing Babel [visitors]. Each visitor function will be called as `(node, state, c)`, where `node` is the AST node, `state` is the same `state` passed to `walk.recursive`, and `c` is a function that takes a single node as argument and continues walking _that_ node. If no visitor for a node is provided, the default walker algorithm will still be used.
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When `walk.recursive` is called with a fresh set of visitors, it will first "explode" the visitors (e.g. expanding `Visitor(node, state) {}` to `Visitor() { enter(node, state) {} }`). This exploding process can take some time, so it is recommended to cache the result of calling `walk.recursive(visitors)` and communicate state leveraging the `state` parameter.
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Unlike other babel-walk walkers, `walk.recursive` does not call the `exit` visitor, only the `enter` (the default) visitor, of a specific node type.
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All [babel-types] aliases (e.g. `Expression`) work, but the union syntax (e.g. `'Identifier|AssignmentPattern'(node, state) {}`) does not.
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In the following example, we are trying to count the number of functions in the outermost scope. This means, that we can simply walk all the statements and increment a counter if it is a function declaration or expression, and then stop walking. Note that we do not specify a visitor for the `Program` node, and the default algorithm for walking `Program` nodes is used (which is what we want). Also of note is how I bring the `visitors` object outside of `countFunctions` so that the object can be cached to improve performance.
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```js
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import * as t from 'babel-types';
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import {parse} from 'babel';
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import * as walk from 'babel-walk';
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const visitors = walk.recursive({
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Statement(node, state, c) {
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if (t.isVariableDeclaration(node)) {
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for (let declarator of node.declarations) {
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// Continue walking the declarator
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c(declarator);
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}
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} else if (t.isFunctionDeclaration(node)) {
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state.counter++;
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}
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},
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VariableDeclarator(node, state) {
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if (t.isFunction(node.init)) {
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state.counter++;
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}
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},
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});
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function countFunctions(node) {
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const state = {
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counter: 0,
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};
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visitors(node, state);
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return state.counter;
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}
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const ast = parse(`
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// Counts
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var a = () => {};
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// Counts
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function b() {
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// Doesn't count
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function c() {
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}
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}
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// Counts
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const c = function d() {};
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`);
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countFunctions(ast);
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// = 3
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```
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[babel-types]: https://github.com/babel/babel/tree/master/packages/babel-types
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[cache your visitors]: https://github.com/thejameskyle/babel-handbook/blob/master/translations/en/plugin-handbook.md#toc-optimizing-nested-visitors
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[visitors]: https://github.com/thejameskyle/babel-handbook/blob/master/translations/en/plugin-handbook.md#toc-visitors
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## Caveat
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For those of you migrating from Acorn to Babel, there are a few things to be aware of.
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1. The visitor caching suggestions do not apply to Acorn's walk module, but do for babel-walk.
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2. babel-walk does not provide any of the other functions Acorn's walk module provides (e.g. `make`, `findNode*`).
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3. babel-walk does not use a `base` variable. The walker algorithm is the same as what babel-traverse uses.
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- That means certain nodes that are not walked by Acorn, such as the `property` property of a non-computed `MemberExpression`, are walked by babel-walk.
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## License
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MIT
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