@kgrz/tslint-lodash

TSLint rules for Lodash

⚠️ Disclaimer: These are in no way official TSLint rules for Lodash, and they do not cover every aspect of the library's usage. Read more about the motivations below.

🚧 Beta alert! This is sub 0.0.1 release, so buyer beware.

Motivation

get-check-undefined:

Due to the changes in how Lodash's TS types (via @types/lodash), and the inherent complexity in defining good types for lodash.get return values, I see the following pattern a lot in code-reviews:

const someObject: string = lodash.get(mainObject, 'some.complicated.path.traversal')
// ...after some more lines of code
const itemLength = someObject.length; // 💥 at runtime, fine at compile time!

In most cases, the programmer who wrote this wanted to use get just to have a safe-traversal on a complex object, but didn't bother to add a default value, perhaps because they were checking for nulls somewhere down the line.

However, this particular line causes subtle bugs in practice. The right-side computation can return undefined if the path in the object doesn't exist, even perhaps due to a typo (not all that uncommon). Until at-least typescript 3.5breaking-changes, due to the way the @types/lodash types are defined, the return type of get when no default parameter (third argument to the call) is specified is set to anylodash-type-any. Due to this, and since we specified the variable value to be a string type, we've potentially masked the case when the right-side call returns undefined ^{^1}.

It's kind of ironic that without TS, that line would've been checked during code-reviews thoroughly. Having TS in the codebase kind of gives a sense of security, and in this case, it ends up providing a false sense of security.

I can't review all the PRs that are opened in our project. So I decided to automate this.

How it works

🚧 As of 0.0.1.beta. version, this plugin looks for any function calls that are lodash.get or get, and checks the following:

1. Is it being assigned to a variable?
2. If so, does it have | undefined in its type definition.

If these two cases match, that line will be marked as an error.

Roadmap

• Add metadata property and specify description
• Add better check for whether the get function is actually from lodash. This could either be done by providing a configuration option listing out regexes, and/or by scanning imports.
• Add tests

Installation

npm install @kgrz/tslint-lodash@beta

Configuration

In your project's tslint.json, add the following:

{
	"extends": [
		"@kgrz/tslint-lodash"
	],
	"rules": {
		"get-check-undefined": true
	}
}

---

^1: It's kind of interesting to note that in TypeScript 3.5 and above, these any types get auto-converted as unknown type, kind of fixing the problem. unknown cannot be converted to a string, and will throw an error.