After more than 20 years of programming, and being a bit seasoned on the TypeScript/JavaScript space, one morning I decided to ask an AI this question:

given that typescript tries to improve javascript, so to speak, wouldn't it be better if devs using typescript stop using undefined, and just use null, to prevent one of the main javascript pitfalls?

The AI replied with something both unexpected and surprising — TL;DR: "nah, rather use undefined instead of null, if you're going to avoid one of them".

Here was the full reply:

While eliminating one of the "nullish" values (either null or undefined) is a popular strategy to simplify code, the consensus in the TypeScript community is not to use only null. In fact, many developers and the TypeScript team itself often prefer using undefined exclusively. [1, 2, 3, 4, 5]

Here is a breakdown of why only using null is not necessarily better, and why undefined is frequently favored. [6, 7]

1. The "Undefined" Nature of JavaScript

undefined is the default, native, and unavoidable "non-value" in JavaScript.

• Uninitialized Variables: If you declare a variable without assigning a value, it is undefined. • Missing Object Properties: Accessing a non-existent property returns undefined. • Function Returns: A function that doesn't explicitly return a value returns undefined. [10, 11]

By trying to use null everywhere, you are forcing a value that isn't native to the language's defaults, leading to, as one commentator put it, "fighting the language". [3]

2. TypeScript and Optionality

In TypeScript, the ? operator (optional properties/parameters) automatically treats missing values as undefined, not null. [10, 12]

interface User {
  id: number;
  phone?: string; // This is equivalent to string | undefined
}

If you adopt a "use null only" policy, you must avoid the ? operator, making your types more verbose (string | null) and increasing the surface area for errors. [13]

3. JavaScript Behaviors (Default Values)

undefined and null behave differently in JS:

• Default Parameters: In functions, default parameters only trigger if the argument is undefined, not null. • JSON Serialization: When using JSON.stringify(), keys with undefined values are removed entirely, while keys with null are kept. [3, 14, 15]

4. The Case for Using Only One

The real pitfall is not using both, but using them inconsistently. Either path —"only null" or "only undefined"— is better than mixing them.

• Pro-Undefined: It is the language default, cleaner with ?, and consistent with modern TypeScript libraries. • Pro-Null: It is more explicit, representing "I intentionally set this to nothing." [3, 10, 17, 18]

5. Best Practice: Embrace Strictness [19]

Instead of banning one, the best practice to avoid "null pitfalls" in TypeScript is:

1. Enable strictNullChecks in tsconfig.json`. This forces you to handle potential / cases at compile time.
2. Use ?. (Optional Chaining) and ?? (Nullish Coalescing) to safely handle both values.
3. Use == null to check for both null and undefined safely when needed. [14, 20, 21]

Summary: While using only null might seem cleaner from a traditional OOD perspective, using only undefined (or accepting both with strict checks) is generally more idiomatic in the TypeScript/JavaScript ecosystem, as it aligns with how the engine itself handles missing data. [1, 2, 6]

AI responses may include mistakes.

[1] https://stackoverflow.com/questions/49746803/typescript-null-versus-undefined

[2] https://schibsted-vend.pl/blog/javascript-dilemma/

[3] https://www.reddit.com/r/typescript/comments/11dpu05/undefined_vs_null/

[4] https://basarat.gitbook.io/typescript/recap/null-undefined

[5] https://www.reddit.com/r/typescript/comments/1aw0c85/how_to_reason_about_possibly_null_values_during/

[6] https://www.reddit.com/r/typescript/comments/11dpu05/undefined_vs_null/

[7] https://www.reddit.com/r/typescript/comments/dq2zla/is_it_bad_practice_to_use_optional_fields_on/

[8] https://www.reddit.com/r/typescript/comments/11dpu05/undefined_vs_null/

[9] https://palantir.github.io/tslint/rules/no-null-keyword/

[10] https://medium.com/@codetheinterface/undefined-vs-null-the-ultimate-guide-for-javascript-and-typescript-dabf9346309a

[11] https://medium.com/@codetheinterface/undefined-vs-null-the-ultimate-guide-for-javascript-and-typescript-dabf9346309a

[12] https://spin.atomicobject.com/optional-undefined-typescript/

[13] https://stackoverflow.com/questions/37980559/is-it-better-to-return-undefined-or-null-from-a-javascript-function

[14] https://basarat.gitbook.io/typescript/recap/null-undefined

[15] https://www.reddit.com/r/learnjavascript/comments/1b3f3s7/is_my_interpretation_of_null_undefined_and_nan/

[16] https://langdev.stackexchange.com/questions/4209/should-a-language-have-both-null-and-undefined-values

[17] https://www.youtube.com/watch?v=SNfjcalqMQE

[18] sindresorhus/eslint-plugin-unicorn#1507

[19] https://www.reddit.com/r/node/comments/3ixp5o/is-it-best-practice_to-use-strict/

[20] https://javascript.plainenglish.io/mastering-null-undefined-in-typescript-optional-chaining-nullish-coalescing-best-practices-b1404be1e77d

[21] https://jameshenry.blog/typescript-null-and-undefined-types/

Fascinating.

Of course, I love the strictNullChecks flag, but being used to the simplicity of FP's languages approach of rather not having null at all, I embarked on the quest of bringing that to TypeScript.

So:

• Not avoiding undefined in favour of null.
• Not avoiding null in favour of undefined, either.
• Not adopting the religious strictNullChecks along with carefully using ?., ?? and == null (as opposed to triple ===), either.

NO! There has to be a better way.

And, no, adopting the HUGELY BLOATED lib 'Effect.ts' is not it. We need something way more simple.

Enter fp-sdk.