skill:jquery-4 — Migrate jQuery 3.x to 4.0.0

Version: 1.0.0

Purpose

Safely migrate jQuery 3.x codebases to jQuery 4.0.0, with first-class support for WordPress themes, plugins, and legacy web applications. This skill systematically audits every breaking change introduced in jQuery 4.0, generates a prioritized migration plan with exact code replacements, and produces safe, reviewable patches.

Use this skill when:

• Upgrading a WordPress site or plugin from jQuery 3.x to 4.0

• Migrating a legacy web application that depends on removed jQuery APIs

• Auditing a codebase for jQuery 4.0 compatibility before upgrading

• Resolving jQuery Migrate plugin warnings in preparation for jQuery 4.0

File Structure

skills/jquery-4/ ├── SKILL.md (this file) └── examples.md

jQuery 4.0 Breaking Changes Reference

Removed Static Methods

Removed API Replacement

$.isArray(x)

Array.isArray(x)

$.trim(str)

str.trim() or String.prototype.trim.call(str)

$.parseJSON(str)

JSON.parse(str)

$.type(obj)

typeof obj or custom type-checking

$.isFunction(fn)

typeof fn === "function"

$.isNumeric(val)

!isNaN(parseFloat(val)) && isFinite(val)

$.isWindow(obj)

obj != null && obj === obj.window

$.now()

Date.now()

$.proxy(fn, ctx)

fn.bind(ctx)

Other Breaking Changes

• Focus event order: focus /blur now fire before focusin /focusout (reversed from 3.x)

• Deferred/Promise alignment: .then() follows native Promise semantics (async resolution, error swallowing changes)

• Slim build removed: Only one build; AJAX and effects are always included

• :even / :odd selectors: Now 0-based (:even selects 1st, 3rd, 5th elements)

• jQuery.holdReady() removed: Use $.when() or native module loading instead

• jQuery.unique() removed: Use jQuery.uniqueSort() instead

• .andSelf() removed: Use .addBack() instead

• .size() removed: Use .length property instead

• jQuery.cssNumber reduced: Several properties removed from the unitless list

Interface References

• Context: Loaded via ContextProvider Interface

• Memory: Accessed via MemoryStore Interface

• Shared Patterns: Shared Loading Patterns

• Schemas: Validated against context_metadata.schema.json and memory_entry.schema.json

Mandatory Workflow

⚠️ IMPORTANT: Execute ALL steps in order. Do not skip any step.

Step 1: Initial Analysis

• Detect jQuery version in use (check package.json , CDN references, wp_enqueue_script calls, bundled files)

• Identify all jQuery plugin dependencies and their jQuery version requirements

• Detect custom jQuery extensions ($.fn.* additions, $.extend usage)

• Determine WordPress context: theme, plugin, mu-plugin, or standalone

• Catalog all JS files that reference jQuery or $

• Check for existing jQuery Migrate plugin usage

Step 2: Load Memory

Follow Standard Memory Loading with skill="jquery-4" and domain="engineering" .

Load project-specific memory:

• migration_progress.md — previous migration state, completed files, blockers

• codebase_patterns.md — known jQuery usage patterns, custom abstractions, plugin inventory

Step 3: Load Context

Follow Standard Context Loading for the engineering domain. Stay within the file budget declared in frontmatter.

Step 4: Audit Breaking Changes

Systematically scan every JS file for jQuery 4.0 incompatibilities:

Removed Static Methods — grep for each removed API:

• $.isArray , jQuery.isArray

• $.trim , jQuery.trim

• $.parseJSON , jQuery.parseJSON

• $.type , jQuery.type

• $.isFunction , jQuery.isFunction

• $.isNumeric , jQuery.isNumeric

• $.isWindow , jQuery.isWindow

• $.now , jQuery.now

• $.proxy , jQuery.proxy

Removed Instance Methods — scan for:

• .andSelf() → .addBack()

• .size() → .length

Removed Utilities:

• jQuery.unique() → jQuery.uniqueSort()

• jQuery.holdReady() → remove or replace with module loading

Behavioral Changes:

• Focus/blur event handlers that depend on event order relative to focusin/focusout

• .then() chains on $.Deferred() that rely on synchronous resolution

• :even / :odd selector usage that assumes 1-based indexing

• Slim build references (jquery.slim.min.js ) that must switch to the full build

WordPress-Specific:

• jQuery.noConflict() usage and $ wrapper patterns

• wp-includes/js/jquery/ bundled version references

• wp_enqueue_script('jquery') dependency chains

Produce a categorized audit report with file paths, line numbers, and severity.

Step 5: Generate Migration Plan

Produce a prioritized, file-by-file migration plan:

• Critical — Code that will break immediately (removed APIs, removed methods)

• High — Behavioral changes that may cause subtle bugs (focus events, Deferred semantics)

• Medium — Deprecated patterns that should be modernized (slim build refs, selector changes)

• Low — Optional improvements (replace jQuery utilities with modern JS equivalents)

For each finding, specify:

• File path and line number(s)

• Current code snippet

• Exact replacement code

• Risk level and testing notes

Step 6: Apply Safe Transformations

Generate code patches/diffs for each file:

• Produce unified diff format for each change

• Group related changes per file

• Include rollback instructions (original code in comments or git revert guidance)

• Handle WordPress-specific patterns:

• Preserve jQuery(document).ready(function($) { ... }) wrapper pattern

• Maintain wp.hooks integration points

• Respect jQuery.noConflict() mode — never use bare $ outside wrapper

Step 7: Validate Migration

Produce a test checklist:

• All removed API calls replaced with native equivalents

• Focus/blur event handlers tested for correct order

• Deferred/Promise chains tested for async behavior changes

• :even /:odd selectors verified for 0-based indexing

• Slim build references updated to full build

• WordPress admin pages load without JS console errors

• Front-end interactions (forms, modals, tabs) function correctly

• Third-party jQuery plugins tested for compatibility

Include jQuery Migrate plugin strategy:

• Install jQuery Migrate 3.x to catch runtime warnings before upgrading

• Run application with Migrate plugin and capture all warnings

• Address all warnings, then remove Migrate and upgrade to jQuery 4.0

Step 8: Generate Output

• Save output to /claudedocs/jquery-4_{project}_{YYYY-MM-DD}.md

• Follow naming conventions in ../OUTPUT_CONVENTIONS.md

• Include: audit report, migration plan, generated patches, test checklist

Step 9: Update Memory

Follow Standard Memory Update for skill="jquery-4" .

Update memory with:

• Migration progress (files completed, files remaining, blockers)

• Discovered codebase patterns (custom jQuery abstractions, plugin versions)

• WordPress-specific findings (theme structure, enqueue patterns)

Compliance Checklist

Before completing, verify:

• All mandatory workflow steps executed in order

• Standard Memory Loading pattern followed (Step 2)

• Standard Context Loading pattern followed (Step 3)

• Every jQuery 4.0 breaking change category audited (Step 4)

• Migration plan includes exact code replacements (Step 5)

• Patches include rollback notes (Step 6)

• Test checklist covers all behavioral changes (Step 7)

• Output saved with standard naming convention (Step 8)

• Standard Memory Update pattern followed (Step 9)

Version History

Version Date Changes

1.0.0 2025-06-27 Initial release — full jQuery 3.x → 4.0 migration support