Logical Grouping & Precedence

Logical grouping is a way to group multiple conditions together to form a single condition. This is useful when you want to apply the same logical operator to multiple conditions. For example, you can group two conditions together and apply the AND operator to them.

• Self Grouping
• Manual Grouping with Parentheses
  • Parser Intervention
• Cross-Apply
  • Advanced Cross-Apply Grouping
• Limitations

Self Grouping

Self grouping expressions is the process of automatically grouping expressions based on their logical operator, or keyword.

The parser will always group AND expressions together. Thus OR operators will determine the split between groups. As described in the combinators section, the parser will also group and treat them as AND operators. Hence why the AND operator is not required when chaining combinators.

Here's an example to illustrate self grouping:

SQL

SELECT * FROM DOM WHERE
TAG = 'a' AND CLASS = 'active'
OR CLASS = 'link'
OR TAG = 'button' AND ID = 'to-top-link'

CSS

a.active, .link, button#to-top-link {
  /* Styles */
}

In this example:

• The SQL query selects elements that are:
  • <a> elements with the class active.
  • Elements with the class link.
  • <button> elements with the ID to-top-link.
• The parser groups the conditions based on the OR operator. Similarly, the above query can also be written as:

SELECT * FROM DOM WHERE
(TAG = 'a' AND CLASS = 'active')
OR CLASS = 'link'
OR (TAG = 'button' AND ID = 'to-top-link')

info

For readability sake, manually define groupings - with parentheses () - where possible.

Manual Grouping with Parentheses

Manual grouping is the process of explicitly grouping expressions together using parentheses (). This is useful when you want to apply a specific logical operator to multiple conditions.

Here's an example to illustrate manual grouping:

SQL

SELECT * FROM DOM WHERE
(TAG = 'a' AND CLASS = 'active')
OR (CLASS = 'link' AND TAG = 'button')

CSS

a.active, button.link {
  /* Styles */
}

In this example:

• The SQL query selects elements that are:
  • <a> elements with the class active.
  • <button> elements with the class link.

Parser Intervention

CSS Selectors are quite basic in nature, and thus attempting 'unique' AND/OR combinations purely based on logical principles driven by the user, may not always work as expected. The parser will intervene and group expressions based on the logical operator precedence. In a way, the parser will always try to ensure that the query is valid and can be parsed correctly in accordance with W3C standards.

Extending from the above example, here's an example to illustrate parser intervention:

SQL

SELECT * FROM DOM WHERE
(
    TAG = 'a' AND CLASS = 'active' -- Removed manual parentheses that closed off the 'AND' grouping
    OR CLASS = 'link' AND TAG = 'button'
)

CSS

a.active, button.link { /* Result remains the same */
  /* Styles */
}

Cross-Apply

Cross-apply is a way to apply a single selector to multiple conditions. This is useful to shorthand most queries and reduce redundancy.

Cross-apply expressions can be achieved by grouping multiple OR expressions, and preceding them with a single AND selector. This will apply the AND selector to each OR expression.

Here's an example to illustrate cross-apply:

SQL

SELECT * FROM DOM WHERE
TAG = 'a' AND (CLASS = 'active' OR CLASS = 'link')

CSS

a.active, a.link {
  /* Styles */
}

SQL Alternative

SELECT * FROM DOM WHERE
TAG = 'a' AND CLASS = 'active'
OR TAG = 'a' AND CLASS = 'link'

In this example:

• The SQL query selects <a> elements that have either the class active or link.
• The CSS selector applies styles to <a> elements with the class active or link.

Advanced Cross-Apply Grouping

Cross-apply can also be used to apply multiple selectors to multiple conditions. This is useful when you have multiple conditions that overlap with each other.

Here's an example to illustrate advanced cross-apply grouping:

SQL

SELECT * FROM DOM WHERE
CLASS = 'orange' AND TAG = 'a' 
AND (
        CLASS = 'active' AND ID = 'new-link'
        OR CLASS = 'link' AND ID = 'old-link'
    )

CSS

a#new-link.orange.active, a#old-link.orange.link {
  /* Styles */
}

SQL Alternative

SELECT * FROM DOM WHERE
CLASS = 'orange' AND TAG = 'a' AND CLASS = 'active' AND ID = 'new-link'
OR CLASS = 'orange' AND TAG = 'a' AND CLASS = 'link' AND ID = 'old-link'

In this example:

• The SQL query selects <a> elements that are:
  • Orange and have the class active and ID new-link.
  • Orange and have the class link and ID old-link.

warning

As of V1.0, cross-apply selectors must always precede the OR expressions. Appending them after an OR expression will result in unexpected behavior.

SQL Invalid

SELECT * FROM DOM WHERE
(CLASS = 'active' OR CLASS = 'link') AND TAG = 'a' -- Cross-apply selector is placed after the OR expression

SQL Valid

SELECT * FROM DOM WHERE
TAG = 'a' AND (CLASS = 'active' OR CLASS = 'link') -- Cross-apply selector is placed before the OR expression

Limitations

The parser currently only supports shallow grouping. Meaning, you can not nest multiple groups within each other.

For example, the following query is invalid and will likely throw a parsing error, or generate invalid CSS selectors:

SELECT * FROM DOM WHERE
TAG = 'a' AND (CLASS = 'active' OR (CLASS = 'link' AND ID = 'new-link'))