postcss-selector-parser

What are postcss-selector-parser's main functionalities?

const parser = require('postcss-selector-parser');
const transform = selectors => {
  selectors.walkTags(tag => {
    if (tag.value === 'h1') {
      tag.value = 'h2';
    }
  });
};
const transformed = parser(transform).processSync('h1.class');

const parser = require('postcss-selector-parser');
const extractClasses = selector => {
  const classes = [];
  parser(selectors => {
    selectors.walkClasses(classNode => {
      classes.push(classNode.value);
    });
  }).processSync(selector);
  return classes;
};
const classes = extractClasses('.class1.class2');

const parser = require('postcss-selector-parser');
const transformPseudo = selector => {
  return parser(selectors => {
    selectors.walkPseudos(pseudo => {
      if (pseudo.value === ':hover') {
        pseudo.value = ':focus';
      }
    });
  }).processSync(selector);
};
const result = transformPseudo('a:hover');

4.0.0

This release has BREAKING CHANGES that were required to fix bugs regarding values with escape sequences. Please read carefully.

• Identifiers with escapes - CSS escape sequences are now hidden from the public API by default. The normal value of a node like a class name or ID, or an aspect of a node such as attribute selector's value, is unescaped. Escapes representing Non-ascii characters are unescaped into unicode characters. For example: bu\tton, .\31 00, #i\2764\FE0Fu, [attr="value is \"quoted\""] will parse respectively to the values button, 100, i❤️u, value is "quoted". The original escape sequences for these values can be found in the corresponding property name in node.raws. Where possible, deprecation warnings were added, but the nature of escape handling makes it impossible to detect what is escaped or not. Our expectation is that most users are neither expecting nor handling escape sequences in their use of this library, and so for them, this is a bug fix. Users who are taking care to handle escapes correctly can now update their code to remove the escape handling and let us do it for them.

• Mutating values with escapes - When you make an update to a node property that has escape handling The value is assumed to be unescaped, and any special characters are escaped automatically and the corresponding raws value is immediately updated. This can result in changes to the original escape format. Where the exact value of the escape sequence is important there are methods that allow both values to be set in conjunction. There are a number of new convenience methods for manipulating values that involve escapes, especially for attributes values where the quote mark is involved. See https://github.com/postcss/postcss-selector-parser/pull/133 for an extensive write-up on these changes.

Upgrade/API Example

In 3.x there was no unescape handling and internal consistency of several properties was the caller's job to maintain. It was very easy for the developer to create a CSS file that did not parse correctly when some types of values were in use.

const selectorParser = require("postcss-selector-parser");
let attr = selectorParser.attribute({attribute: "id", operator: "=", value: "a-value"});
attr.value; // => "a-value"
attr.toString(); // => [id=a-value]
// Add quotes to an attribute's value.
// All these values have to be set by the caller to be consistent:
// no internal consistency is maintained.
attr.raws.unquoted = attr.value
attr.value = "'" + attr.value + "'";
attr.value; // => "'a-value'"
attr.quoted = true;
attr.toString();  // => "[id='a-value']"

In 4.0 there is a convenient API for setting and mutating values that may need escaping. Especially for attributes.

const selectorParser = require("postcss-selector-parser");

// The constructor requires you specify the exact escape sequence
let className = selectorParser.className({value: "illegal class name", raws: {value: "illegal\\ class\\ name"}});
className.toString(); // => '.illegal\\ class\\ name'

// So it's better to set the value as a property
className = selectorParser.className();
// Most properties that deal with identifiers work like this
className.value = "escape for me";
className.value; // => 'escape for me'
className.toString(); // => '.escape\\ for\\ me'

// emoji and all non-ascii are escaped to ensure it works in every css file.
className.value = "😱🦄😍";
className.value; // => '😱🦄😍'
className.toString(); // => '.\\1F631\\1F984\\1F60D'

// you can control the escape sequence if you want, or do bad bad things
className.setPropertyAndEscape('value', 'xxxx', 'yyyy');
className.value; // => "xxxx"
className.toString(); // => ".yyyy"

// Pass a value directly through to the css output without escaping it. 
className.setPropertyWithoutEscape('value', '$REPLACE_ME$');
className.value; // => "$REPLACE_ME$"
className.toString(); // => ".$REPLACE_ME$"

// The biggest changes are to the Attribute class
// passing quoteMark explicitly is required to avoid a deprecation warning.
let attr = selectorParser.attribute({attribute: "id", operator: "=", value: "a-value", quoteMark: null});
attr.toString(); // => "[id=a-value]"
// Get the value with quotes on it and any necessary escapes.
// This is the same as reading attr.value in 3.x.
attr.getQuotedValue(); // => "a-value";
attr.quoteMark; // => null

// Add quotes to an attribute's value.
attr.quoteMark = "'"; // This is all that's required.
attr.toString(); // => "[id='a-value']"
attr.quoted; // => true
// The value is still the same, only the quotes have changed.
attr.value; // => a-value
attr.getQuotedValue(); // => "'a-value'";

// deprecated assignment, no warning because there's no escapes
attr.value = "new-value";
// no quote mark is needed so it is removed
attr.getQuotedValue(); // => "new-value";

// deprecated assignment, 
attr.value = "\"a 'single quoted' value\"";
// > (node:27859) DeprecationWarning: Assigning an attribute a value containing characters that might need to be escaped is deprecated. Call attribute.setValue() instead.
attr.getQuotedValue(); // => '"a \'single quoted\' value"';
// quote mark inferred from first and last characters.
attr.quoteMark; // => '"'

// setValue takes options to make manipulating the value simple.
attr.setValue('foo', {smart: true});
// foo doesn't require any escapes or quotes.
attr.toString(); // => '[id=foo]'
attr.quoteMark; // => null 

// An explicit quote mark can be specified
attr.setValue('foo', {quoteMark: '"'});
attr.toString(); // => '[id="foo"]'

// preserves quote mark by default
attr.setValue('bar');
attr.toString(); // => '[id="bar"]'
attr.quoteMark = null;
attr.toString(); // => '[id=bar]'

// with no arguments, it preserves quote mark even when it's not a great idea
attr.setValue('a value \n that should be quoted');
attr.toString(); // => '[id=a\\ value\\ \\A\\ that\\ should\\ be\\ quoted]'

// smart preservation with a specified default
attr.setValue('a value \n that should be quoted', {smart: true, preferCurrentQuoteMark: true, quoteMark: "'"});
// => "[id='a value \\A  that should be quoted']"
attr.quoteMark = '"';
// => '[id="a value \\A  that should be quoted"]'

// this keeps double quotes because it wants to quote the value and the existing value has double quotes.
attr.setValue('this should be quoted', {smart: true, preferCurrentQuoteMark: true, quoteMark: "'"});
// => '[id="this should be quoted"]'

// picks single quotes because the value has double quotes
attr.setValue('a "double quoted" value', {smart: true, preferCurrentQuoteMark: true, quoteMark: "'"});
// => "[id='a "double quoted" value']"

// setPropertyAndEscape lets you do anything you want. Even things that are a bad idea and illegal.
attr.setPropertyAndEscape('value', 'xxxx', 'the password is 42');
attr.value; // => "xxxx"
attr.toString(); // => "[id=the password is 42]"

// Pass a value directly through to the css output without escaping it. 
attr.setPropertyWithoutEscape('value', '$REPLACEMENT$');
attr.value; // => "$REPLACEMENT$"
attr.toString(); // => "[id=$REPLACEMENT$]"