Add TypeScript type checking - #6862

Summary

Wanted to try using TypeScript with allowJs and checkJs to see if it can improve code correctness.

Webpack is a good case study for using TypeScript compiler on JavaScript projects. This is still a work in progress and I'm not sure if we can use TypeScript to check for errors without any code modifications.

Issues in TypeScript and DefiantlyTyped

• TypeScript does not allow using _this as a variable name.
  • added collision check for '_this' microsoft/TypeScript#155.
  • Error: Expression resolves to variable declaration '_this' that compiler uses to capture 'this' reference.
  • variable name _this breaks in checkJs microsoft/TypeScript#22866
• Assigning properties to class members is not allowed. even in non strict mode
  • TypeScript does not like this code:
  • class Foo {}; let f = new Foo(); f.bar = 1
  • In JS, assignment to an undeclared instance property should act as its declaration microsoft/TypeScript#22896
  • We will initialize members with undefined as value in class constructor to surpass those errors
  • Initialize fileTimestamps and contextTimestamps in Compilation #6883
  • Initialize useSourceMap in Module #6886
  • Initialize compilationDependencies in Compilation as undefined #6885
  • Initialize loc member in Dependency #6884
  • Initialize start and end time as undefined in Stats #6897
  • Add missing class member initializations found by TypeScript #6910
  • Refactor WebpackError to initialize details as undefined #6974
  • Add class member initializations for multiple classes #6987
• Some @types/tappable types are not accurate
  • Add MultiHook and _pluginCompat to Tapable  DefinitelyTyped/DefinitelyTyped#24527
• TypeScript confusing class prototype assignments with instance members
  • Errors like Class 'Module' defines instance member property 'identifier', but extended class 'ExternalModule' defines it as instance member function.
  • Extending a class method with prototype assignment confuses the compiler about membership type microsoft/TypeScript#22895
• Webpack is using some undocumented and deprecated Node.js APIs
  • Add _nodeModulePaths to node Module class DefinitelyTyped/DefinitelyTyped#24526
  • Add deprecated process.bindings to node.js  DefinitelyTyped/DefinitelyTyped#24521
  • Those definitions can be declared in declarations.d.ts
  • In one case we had to use // @ts-ignore directive
• TypeScript is too strict about module.exports assignments
  • checkJs too strict about module.exports assignment microsoft/TypeScript#15647
• Type definitions for vm.runInThisContext is wrong. it does not accept string filename for options
  • Add string option for vm.runIn*Context methods in Node.js typing DefinitelyTyped/DefinitelyTyped#24798
• TypeScript can not understand some of existing JSDoc types in Webpack
  • Fix JSDoc for makeNewProfiledTapFn to conform to TypeScript types #6988
  • Intellisense for object properties defined in multi-line JSDoc comments microsoft/TypeScript#11597 (comment)
• TypeScript does not like the fact that a subclass static member is not conforming to the superclass
  • Make HarmonyAcceptImportDependencyTemplate extend HarmonyImportDependency.Template #6989
• TypeScript does not like overriding values with different types and there is no way to have another type than string for iterator of for..in in Object.keys
  • Avoid type mismatch assignment of number to string in HotModuleReplacementPlugin #7020

Issues found in Webpack by TypeScript

• 1. Accessing compilationDependencies in Compilation (Fix: Initialize compilationDependencies in Compilation as undefined #6885)
• 2. Passing 3 args to WebAssemblyImportDependency Remove extra argument from WebAssemblyImportDependency constructor call site #6882
• 3. JavascriptGenerator has no constructor but getting arguments Remove extra argument passed to JavascriptGenerator constructor #6881
• 4. setProperty with too many args Remove extraneous argument from setOptions call sites in OptionsDefaulter #6875 Remove extra argument passed to getEntrypointSize in SizeLimitsPlugin #6900 Remove extra argument sent to HarmonyExportExpressionDependencyConcat… #6899 Remove extra argument passed to ContextModuleFactory in Compiler #6898
• 5. hideStack is being appended to Error class.
  • Fix Add HarmonyLinkingError class #6932
• 6. Unused dependencyTemplatesHashMap weak map Use a Map for dependencyTemplatesHashMap instead of a WeakMap #6922
• 7. Using Array methods on Set Use add() to append items to activeExports set in HarmonyExportImporedSpecifierDependency #6950
• 8. Webpack has some invalid calls to Date and JSON APIs JSON and Date API usage fixes #6982
• 9. cacheMerged will not work with more than 2 arguments
  • TS2448 used before its declaration error for recursive function  microsoft/TypeScript#22975
  • Simplify cachedMerge #7013 (Thanks to @ooflorent!)

When this is complete?

This is going to be a lot of work to get it to the finish line and I'm not sure if it is inline with the project goals. This is why I'm opening the pull request early. Hoping we can overcome all of those TypeScript issues and make use of TypeScript compiler in Webpack! 🙏

Only comment changes

This pull request on its own should not hav any code changes. TypeScript type checking should be non-invasive and should not make users to change their code. At most, changes should be comments only. If any code change is necessary due to bugs or TS limitations, we are going to propose and merge them in separate pull request.

TypeScript 2.9

The amazing TypeScript team have fixed many issues with the compiler when checking JavaScript code. Those changes are currently (April 11) in the nightly builds of version 2.9. We can merge while depending on nightly version but it would be nicer to depend to beta release in master. We should eventually depend on a stable release.

@TheLarkInn @DanielRosenwasser @mhegazy