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Category: JavaScript

Type Driven Development: Property Tests
Part 11 – Property Tests

This is a series of posts I’m writing about using types as another tool in software development, Continuous Delivery, & keeping LLM’s honest. They’re also a design & refactoring tool, a communication tool, and reduce how many tests you have to write.
Parts
1. Part 1 – Branded Types
2. Part 2 – Product Types
3. Part 3 – Union & Discriminated Unions
4. Part 4 – Non-Empty Collections
5. Part 5 – Indexed Types
6. Part 6 – unknown vs any
7. Part 7 – Result
8. Part 8 – Schema
9. Part 9 – Total Function
10. Part 10 – Errors as Values
11. Part 11 – Property Tests
We’ve got more than enough type knowledge to build total functions. Type safety is a spectrum; is there a way to determine how total our functions are? A few. One particular way is to utilize Property Tests. You write a single unit test, it generates 100 random inputs. If one of those inputs fails, it’ll give you the random seed so you can test the exact sequence of inputs yourself (e.g. fast-check, jsverify):
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July 2, 2026
Type Driven Development: Errors as Values
Part 10 – Errors as Values

This is a series of posts I’m writing about using types as another tool in software development, Continuous Delivery, & keeping LLM’s honest. They’re also a design & refactoring tool, a communication tool, and reduce how many tests you have to write.
Parts
1. Part 1 – Branded Types
2. Part 2 – Product Types
3. Part 3 – Union & Discriminated Unions
4. Part 4 – Non-Empty Collections
5. Part 5 – Indexed Types
6. Part 6 – unknown vs any
7. Part 7 – Result
8. Part 8 – Schema
9. Part 9 – Total Function
10. Part 10 – Errors as Values
11. Part 11 – Property Tests
Errors as Values moves errors from a random, runtime consequence to an iterative design process using types. u intentionally think about & design what could go wrong w/your code in functions, libraries/back-ends u integrate with & your application. Let’s fix fetch to see how this all works.
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July 1, 2026
Type Driven Development: Total Function
Part 9 – Total Function

This is a series of posts I’m writing about using types as another tool in software development, Continuous Delivery, & keeping LLM’s honest. They’re also a design & refactoring tool, a communication tool, and reduce how many tests you have to write.
Parts
Type safety is a spectrum. A gradual type system widens the spectrum in a good & bad way; u have flexibility to discover your domain as it emerges w/less strict types + integrate w/less typed & legacy systems. That has a cost in safety. Sometimes u can narrow what’s allowed through types, then expand to other possibilities as you learn more. Whatever you choose, aim to design Total Functions; every input returns a sensible value.
(more…)
June 30, 2026
Type Driven Development: Schema
Part 8 – Schema

This is a series of posts I’m writing about using types as another tool in software development, Continuous Delivery, & keeping LLM’s honest. They’re also a design & refactoring tool, a communication tool, and reduce how many tests you have to write.
Parts
To do anything of value in software, we have to get unsafe data from outside our type system into it. We saw how unknown forces us to type narrow, but type narrowing is dangerous, tedious, and it’s hard to be thorough.
(more…)
June 30, 2026

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Software, Fitness, and Gaming – Jesse Warden

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