LWC breaking changes wishlist

[nolanlawson]

Breaking changes that we want to introduce someday.

Changes that only affect npm package consumers (not LEX component authors)

• Expose consistent dist files across packages #3445
• drop non-ESM builds (Remove non-ESM non-modern JS dist files #2430)
• unify compiler and engine packages (Unify compiler and engine packages #2996)
• Specify the minimum supported version of Node in engines #4150
• Use node: protocol for all builtin imports (fs, path, etc.) #4149
• drop runtimeFlags export from @lwc/features (see discussion)
• remove the deprecated transform() API in @lwc/compiler
• remove the deprecated minify option
• use "type": "module" in our Node packages (Drop CommonJS support in Node packages, use "type": "module" #3017)
• Update dependencies that require ESM #3451
• Use Node package exports #2765
• Have the lwc package directly export types from @lwc/engine-dom #3598
• Consider removing setHooks #4442

HTML template compiler changes

• throw error instead of warn on deprecated parse5 errors ([Breaking change] Throw error rather than warning for parse5 HTML errors #3258) (e.g. </template></template>)
• Lowercase all CSS scope tokens #3313
• (template-compiler) Using multiple if:true|false directives on same tag should be a compile-time error #3320
• Disallow using key outside an iteration #3375
• Rendering list items without a key is possible using text/comment nodes #3860
• Normalize whitespace for attributes (breaking change) #4124
• Inconsistent attribute ordering between static/dynamic nodes #4174
• Make duplicate slots in light DOM behave like native shadow DOM slots, or disallow duplicates entirely #4497

CSS compiler changes

• incorrect handling of :host() CSS pseudo in scoped styles (:host(.foo, .bar) is styled differently in synthetic shadow and scoped light DOM vs native shadow #3225)
• Move validation of <style> contents to compilation #3454

JS compiler changes

• Remove Babel object spread transformation #3577
• Remove Babel class properties transformation #3537
• remove dynamic import transformation from Babel plugin component
• remove dedupe imports transformation from Babel plugin component
• remove unnecessary registerDecorators from Babel plugin component ([JS compiler] remove registerDecorators for non-LightningElement classes #2701)
• Use native decorators (for @api, @track, and @wire))
• Throw error on invalid @track/@api fields #3408
• @wire on getter/setter ignores implementation #4450

Module resolution changes

• Remove support for deprecated 'wire-service' package #3517

Runtime changes

• drop no-longer-needed feature flags
• drop "experimental" APIs (getComponentDef, isComponentConstructor, readonly, register, unwrap, etc.)
• freeze the template object instead of just logging a warning (Freeze the template object #2826)
• remove the deprecated buildCustomElementConstructor API
• remove the deprecated wired element host and params metadata
• remove ARIA reflection global polyfill (ARIA reflection polyfill contains non-standard props #2733)
• remove non-standard ARIA props from LightningElement, BridgeElement, and template compiler (ARIA reflection polyfill contains non-standard props #2733)
• use native custom element lifecycle callbacks (Use native connectedCallback/disconnectedCallback lifecycle hooks #3198)
• Deprecate this.template in favor of this.shadowRoot #3344
• feat: use fragments for slots in light DOM (BREAKING CHANGE) #3308
• Public properties with duplicate getter/setter should be an error #3436
• [@engine-core] Inconsistent disconnectedCallback invocation order #2713
• Convert warnings logged from Unexpected LWC stylesheet content to errors #3447
• null/undefined value does not remove xlink:/xml: attribute #3502
• Inconsistent rendering of empty class/style in static vs dynamic nodes #3548
• Components should not "inherit" @api from their superclasses #3762
• Remove support for legacy stylesheet scope tokens #3733
• List re-rendering causes DOM state to be lost #4291
• Synthetic custom element lifecycle swallows DOM errors for insertBefore #4319

Browser compat changes

• drop compat / IE11 support (fix: remove compat (BREAKING CHANGE) #2948 fix: remove legacy browser hacks (BREAKING CHANGE) #2954)
• update browser support matrix
• bump ES2018 build target to ESNext (Bump ES2018 build target to ES2019+/ESNext #3366)
• make ARIA reflection consistent across browsers (ARIA reflection polyfill should treat setting undefined the same as null #3284)
• Deprecate customProperties config from @lwc/style-compiler #3614

SSR changes

• [SSR] Serialized attribute should be shadowrootmode, not shadowroot #3900