docs: ADR for Typescript Guidance

Author: marlonkeating

docs/0003-typescript-guidance.md

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+# Introducing TypeScript
+
+## Status
+Accepted
+
+## Context
+TypeScript is a strongly-typed superset of JavaScript that adds optional static type checking, class-based object-oriented programming, and other features to JavaScript.  
+Here are some of the advantages of TypeScript over JavaScript:
+
+### Type safety
+TypeScript helps catch errors at compile-time instead of runtime by adding type annotations to variables, functions, and classes. This can help prevent errors that might occur when dealing with large codebases.
+### Better tooling support
+TypeScript has better tooling support than JavaScript, with features like code navigation, auto-completion, and refactoring tools in popular code editors like Visual Studio Code.
+### Improved code organization
+TypeScript's class-based object-oriented programming model allows developers to organize code in a more structured and maintainable way.
+### Easy adoption
+TypeScript is a superset of JavaScript, which means that any valid JavaScript code is also valid TypeScript code. This makes it easy for developers to adopt TypeScript gradually, without needing to rewrite their entire codebase.
+### Community support
+TypeScript has a growing community of developers who contribute to its development, create libraries and tools, and provide support to other developers. This makes it easier for developers to learn and adopt TypeScript.
+
+## Decision
+### Target Goals
+Prioritize using TypeScript in the following places:
+* New code files
+* Existing API endpoints (and their payloads)
+* Components or Functions take a lot of parameters, or use parameters that are themselves complex objects
+
+Less important are:
+* React components without complex input objects (especially since they are often adequately served with PropTypes)
+* Test code
+
+### Migration Strategy
+While migrating Javascript code to TypeScript code is relatively straightforward, the initial introduction of TypeScript into a code base can involve some unexpected wrinkles.  While we will 
+work to streamline our infrastructure to make this easier over time, we advise introducing TypeScript in a minimalist fashion initially, by introducing or migrating a single file into TypeScript (or even a single function within a file).  
+
+The reasons for this are practical:
+* The most difficult part of the initial process of introducing TypeScript into a Javascript codebase is harmonizing with the existing build, test, and lint configuration.
+* The smaller the initial effort of introducing TypeScript into a codebase, the easier it is to actually implement around more pressing feature work.
+
+## Consequences
+### Benefits
+* Code that requires heavy contracts, whether that's functions/components with lots or parameters, or complex objects returned from backend API's, will become much more comprehensible and easier to work with in a modern programming IDE.
+* Because TypeScript is a superset of Javascript, the code does not need to be migrated all at once, but can be updated to TypeScript during the course of regular feature work.
+
+### Challenges
+* Migrating to TypeScript falls into the category of Tech Debt, which is difficult to get around to around the demands of mission-critical feature work and bug fixes.
+* There can be a bit of a learning curve around using TypeScript, particularly for Javascript developers who don't have prior experience with strongly-typed languages. 
+
+## References
+* https://www.typescriptlang.org/