This rule raises an issue when array methods like map(), filter(), find(), etc. are called with a thisArg parameter.

Why is this an issue?

JavaScript array methods accept an optional thisArg parameter that sets the this context for the callback function. However, using this parameter often leads to confusion and less readable code.

There are several problems with using thisArg:

Arrow functions ignore thisArg: Arrow functions don’t have their own this context and cannot be bound to a different this value. When you pass a thisArg with an arrow function, it has no effect, making the code misleading.

Regular functions become less explicit: When using regular functions with thisArg, the relationship between the callback and the context object becomes less clear. Readers must understand that the second parameter affects the this binding inside the callback.

Better alternatives exist: Modern JavaScript provides clearer ways to access external values in callbacks. You can directly reference variables from the outer scope, use explicit binding with .bind(), or restructure the code to avoid this altogether.

Using thisArg makes code harder to understand and maintain, especially for developers who may not be familiar with JavaScript’s complex this binding rules.

What is the potential impact?

Using thisArg unnecessarily can lead to:

• Bugs: When arrow functions are used with thisArg, developers might expect the this binding to work, but it doesn’t
• Confusion: Code becomes harder to read and understand, especially for team members less familiar with JavaScript’s this behavior
• Maintenance issues: Future modifications might break the intended this binding without clear indication

How to fix?

Remove the unnecessary thisArg parameter when using arrow functions, since arrow functions ignore the this binding.

Non-compliant code example

const results = items.map(item => transform(item), context); // Noncompliant

Compliant code example

const results = items.map(item => transform(item));

Documentation

• eslint-plugin-unicorn - Rule no-array-method-this-argument
• MDN - Array.prototype.map() - Documentation for the map() method and its thisArg parameter
• MDN - Arrow functions - Explanation of how arrow functions handle the 'this' keyword
• MDN - Function.prototype.bind() - Documentation for explicit function binding as an alternative to thisArg