Printer

Prints Babel AST to readable JavaScript. Use 🐊Putout to parse your code.

You may also use Babel 8 with estree-to-babel for ESTree and Babel AST to put .extra.raw to .raw (which is simpler for transforms, no need to use Optional Chaining and add extra values every time).

• ☝️ Similar to Recast, but twice faster, also simpler and easier in maintenance, since it supports only Babel.
• ☝️ As opinionated as Prettier, but has more user-friendly output and works directly with AST.
• ☝️ Like ESLint but works directly with Babel AST.
• ☝️ Easily extendable with help of Overrides.

Supports:

• ✅ ES2023;
• ✅ JSX;
• ✅ TypeScript;
• ✅ JSON;

Install

npm i @putout/printer

🐊 Support of Printer

Printer has first class support from 🐊Putout with help of @putout/plugin-printer. So install:

1npm i @putout/plugin-printer -aD

And update .putout.json:

1{
2    "printer": "putout",
3    "plugins": ["printer"]
4}

To benefit from it.

Example

1const {print} = require('@putout/printer');
2const {parse} = require('putout');
3const ast = parse('const a = (b, c) => {const d = 5; return a;}');
4
5print(ast);
6// returns
7`
8const a = (b, c) => {
9    const d = 5;
10    return a;
11};
12`;

Overrides

When you need to extend syntax of @putout/printer just pass a function which receives:

• path, Babel Path
• print, a function to output result of printing into token array;

When path contains to dashes __ and name, it is the same as: write(path.get('right')), and this is actually traverse(path.get('right')) shortened to simplify read and process.

Here is how you can override AssignmentPattern:

1const ast = parse('const {a = 5} = b');
2
3print(ast, {
4    format: {
5        indent: '    ',
6        newline: '\n',
7        space: ' ',
8        splitter: '\n',
9        quote: `'`,
10        endOfFile: '\n',
11    },
12    semantics: {
13        comments: true,
14        maxSpecifiersInOneLine: 2,
15        maxElementsInOneLine: 3,
16        maxElementLengthInOneLine: 10,
17        maxLogicalsInOneLine: 3,
18        maxVariablesInOneLine: 4,
19        maxTypesInOneLine: 3,
20        maxPropertiesInOneLine: 2,
21        maxPropertiesLengthInOneLine: 15,
22        trailingComma: true,
23        escapeSingleQuote: true,
24        escapeDoubleQuote: false,
25        roundBraces: {
26            arrow: true,
27            sequence: true,
28            assign: false,
29            new: true,
30        },
31    },
32    visitors: {
33        AssignmentPattern(path, {print}) {
34            print('/* [hello world] */= ');
35            print('__right');
36        },
37    },
38});
39
40// returns
41'const {a/* [hello world] */= 5} = b;\n';

format

Options related to visuals and not related to logic of output can be changed with help of format, you can override next options:

1const overrides = {
2    format: {
3        indent: '    ',
4        newline: '\n',
5        space: ' ',
6        splitter: '\n',
7        endOfFile: '\n',
8    },
9};

• indent - use two spaces, tabs, or anything you want;
• newline - symbol used for line separation;
• space - default symbol used for space character;
• splitter - mandatory symbol that used inside of statements like this:

Default options produce:

1if (a > 3)
2    console.log('ok');
3else
4    console.log('not ok');

But you can override them with:

1const overrides = {
2    format: {
3        indent: '',
4        newline: '',
5        space: '',
6        splitter: ' ',
7    },
8};

And have minified code:

if(a>3)console.log('ok');else console.log('not ok');

Semantics

Options used to configure logic of output, similar to ESLint rules:

• ✅ maxElementsInOneLine - count of ArrayExpression and ArrayPattern elements placed in one line.
• ✅ maxLogicalsInOneLine - count of LogicalExpression elements placed in one line.
• ✅ maxVariablesInOneLine - count of VariableDeclarators in one line.
• ✅ maxPropertiesInOneLine - count of ObjectProperties in one line.
• ✅ maxPropertiesLengthInOneLine - maximum length of Object Property, when violated splits event if maxPropertiesInOneLine satisfies;
• ✅ roundBraces to output braces or not
  • arrow: In a single argument arrow function expressions enabled: (a) => {}, disabled: a => {};
  • sequence: In sequence expressions: enabled: for(let e of l) (a(), b()), disabled: for(let e of l) a(), b();
  • assign: In assignment expressions: enabled: (e.o=w(e.o), disabled: e.o=w(e.o);
  • new: In new expressions: enabled: new Date(), disabled: new Date;

Visitors API

When you want to improve support of existing visitor or extend Printer with a new ones, you need next base operations:

override

When you need to override behavior of existing visitor use:

1import {
2    print,
3    visitors as v,
4} from '@putout/printer';
5
6print(ast, {
7    visitors: {
8        CallExpression(path, printer, semantics) {
9            const {print} = printer;
10            
11            if (!path.node.goldstein)
12                return v.CallExpression(path, printer, semantics);
13            
14            print('__goldstein');
15        },
16    },
17});

print

Used in previous example print can be used for a couple purposes:

• to write string;
• to write node when object passed;
• to write node when string started with __;

1print(ast, {
2    visitors: {
3        AssignmentPattern(path, {print, maybe}) {
4            maybe.write.newline(path.parentPath.isCallExpression());
5            print('/* [hello world] */= ');
6            print('__right');
7        },
8    },
9});

maybe

When you need some condition use maybe. For example, to add newline only when parent node is CallExpression you can use maybe.write.newline(condition):

1print(ast, {
2    visitors: {
3        AssignmentPattern(path, {write, maybe}) {
4            maybe.write.newline(path.parentPath.isCallExpression());
5            write(' /* [hello world] */= ');
6            write('__right');
7        },
8    },
9});

write

When you going to output string you can use low-level function write:

1print(ast, {
2    visitors: {
3        BlockStatement(path, {write}) {
4            write('hello');
5        },
6    },
7});

indent

When you need to add indentation use indent, for example when you output body, you need to increment indentation, and then decrement it back:

1print(ast, {
2    visitors: {
3        BlockStatement(path, {write, indent}) {
4            write('{');
5            indent.inc();
6            indent();
7            write('some;');
8            indent.dec();
9            write('{');
10        },
11    },
12});

traverse

When you need to traverse node path, you can use traverse:

1print(ast, {
2    visitors: {
3        AssignmentExpression(path, {traverse}) {
4            traverse(path.get('left'));
5        },
6    },
7});

This is the same as print('__left') but more low-level, and supports only objects.

Speed Comparison

About speed, for file speed.js:

1const {readFileSync} = require('node:fs');
2
3const putout = require('putout');
4const parser = require('@babel/parser');
5
6const code = readFileSync('./lib/tokenize/tokenize.js', 'utf8');
7const ast = parser.parse(code);
8
9speed('recast');
10speed('putout');
11
12function speed(printer) {
13    console.time(printer);
14    
15    for (let i = 0; i < 1000; i++) {
16        putout(code, {
17            printer,
18            plugins: ['remove-unused-variables'],
19        });
20    }
21    
22    console.timeEnd(printer);
23}

With contents of tokenize.js, we have:

License

MIT