transform the ast on a recursive walk

Usage no npm install needed!

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  import falafelHarmony from 'https://cdn.skypack.dev/falafel-harmony';



Transform the ast on a recursive walk.

browser support

build status

This module is like burrito, except that it uses esprima instead of uglify for friendlier-looking ast nodes.

falafel döner



Put a function wrapper around all array literals.

var falafel = require('falafel');

var src = '(' + function () {
    var xs = [ 1, 2, [ 3, 4 ] ];
    var ys = [ 5, 6 ];
    console.dir([ xs, ys ]);
} + ')()';

var output = falafel(src, function (node) {
    if (node.type === 'ArrayExpression') {
        node.update('fn(' + node.source() + ')');


(function () {
    var xs = fn([ 1, 2, fn([ 3, 4 ]) ]);
    var ys = fn([ 5, 6 ]);
    console.dir(fn([ xs, ys ]));

custom keyword

Creating custom keywords is super simple!

This example creates a new beep keyword that uppercases its arguments:

var falafel = require('falafel');
var src = 'console.log(beep "boop", "BOOP");';

function isKeyword (id) {
    if (id === 'beep') return true;

var output = falafel(src, { isKeyword: isKeyword }, function (node) {
    if (node.type === 'UnaryExpression'
    && node.keyword === 'beep') {
            'String(' + node.argument.source() + ').toUpperCase()'

Now the source string console.log(beep "boop", "BOOP"); is converted to:

$ node example/keyword.js
console.log(String("boop").toUpperCase(), "BOOP");

which we can execute:

$ node example/keyword.js | node



var falafel = require('falafel')

falafel(src, opts={}, fn)

Transform the string source src with the function fn, returning a string-like transformed output object.

For every node in the ast, fn(node) fires. The recursive walk is a pre-traversal, so children get called before their parents.

Performing a pre-traversal makes it easier to write nested transforms since transforming parents often requires transforming all its children first.

The return value is string-like (it defines .toString() and .inspect()) so that you can call node.update() asynchronously after the function has returned and still capture the output.

Instead of passing a src you can also use opts.source.

All of the opts will be passed directly to esprima except for 'range' which is always turned on because falafel needs it.

Some of the options you might want from esprima includes: 'loc', 'raw', 'comment', 'tokens', and 'tolerant'.

falafel uses a custom patch of esprima with support for an opts.isKeyword() function. When opts.isKeyword(id) returns true, the string id will be treated as a keyword. You can use this behavior to create custom unary expression keywords.

An opts.isKeyword(id) value that is a string will be mapped to existing types. The only currently supported string value is "block".


Aside from the regular esprima data, you can also call some inserted methods on nodes.

Aside from updating the current node, you can also reach into sub-nodes to call update functions on children from parent nodes.


Return the source for the given node, including any modifications made to children nodes.


Transform the source for the present node to the string s.

Note that in 'ForStatement' node types, there is an existing subnode called update. For those nodes all the properties are copied over onto the node.update() function.


Reference to the parent element or null at the root element.


With npm do:

npm install falafel