Hey there! Upgrading a large Ext JS 4.2 app to 7.x is no small feat, but breaking it into incremental, focused steps will make the process way more manageable. I’ve helped teams tackle this exact upgrade, so let’s dive into the practical strategies that actually work:

Before touching any code, get clear on what you’re working with:

• Map out all custom components, third-party plugins, and legacy extensions your app relies on. A lot of 4.2-era plugins won’t play nice with 7.x, so flag these for replacement or rewrite early.
• Run the Sencha Cmd Upgrade Analyzer with sencha app upgrade --analyze on your existing 4.2 project. This tool scans your codebase to highlight deprecated classes, methods, and configs specific to your app — way more targeted than generic docs.

Skipping versions is a surefire way to hit a wall. Take it step by step:

• First upgrade to Ext JS 5.1 — it’s the closest bridge to 4.2, and Sencha’s upgrade tools are far more reliable for small jumps.
• Fix all deprecation warnings here before moving on. Key areas to focus on:
  • Store behavior: autoLoad: true was default in 4.2; it’s false in 5.x+. You’ll need to explicitly set it or call store.load() manually where needed.
  • Component lifecycle: initComponent still works, but start getting comfortable with apply/update methods for config handling — they’re the standard in newer versions.

Once 5.1 is stable, upgrade to Ext JS 6.7 (the last 6.x release). This will minimize the final leap to 7.x:

• Toolkit choice: If your app is desktop-only, stick with the classic toolkit initially. You can refactor to the modern toolkit later if you need mobile support.
• MVVM migration: 6.x heavily promotes View Models and View Controllers. If your 4.2 app uses old-school MVC, start migrating components to use viewModel and viewController instead of direct component references. This will make the 7.x transition way smoother.
• Layout updates: Replace outdated layouts like anchor with vbox/hbox using flex where possible — it’s more consistent with 7.x’s layout system.

When your 6.7 app runs without major issues, pull the trigger on 7.x (I recommend 7.6 for stability, but pick the patch version that fits your browser support needs):

• Theming Overhaul: 7.x uses a completely revamped theming system. Your old 4.2 custom themes won’t work — start with the default Triton or Neptune theme, then layer your custom styles using the new Sass variables and mixins.
• Removed Classes: Some 4.2 classes are gone entirely (e.g., Ext.grid.feature.GroupingSummary is now Ext.grid.plugin.GroupingSummary). Use the upgrade analyzer’s report to track these down and replace them.
• ES6+ Support (Optional): 7.x supports modern JS features, so you can refactor old Ext.define patterns to ES6 classes if you want, but this isn’t mandatory — your existing code will still run as long as it avoids deprecated syntax.

Don’t try to test the whole app at once — that’s a recipe for burnout:

• Split your app into modules and test one at a time, starting with the most critical features.
• Use Sencha Test to automate unit tests for core components. If you don’t have existing tests, write basic ones for key parts of your app to catch regressions early.
• Browser support note: 7.x drops IE11 support in versions after 7.2. If you need IE11 support, stick to 7.0-7.2.

• Component Query Syntax: 5.x+ tightened up component query rules — avoid ambiguous selectors like just button. Use #myButton (ID) or [cls='custom-btn'] (class) for more reliable results.
• Plugin Registration: In 4.2, you could often use plugin strings (e.g., plugins: ['groupingsummary']). In 7.x, using config objects like plugins: { type: 'groupingsummary' } is safer and avoids compatibility issues.
• Data Package Changes: 5.x+ made changes to how models and associations work. Double-check that your data loading and association logic still behaves as expected.

内容的提问来源于stack exchange，提问作者DurkoRurko