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It may help to start by thinking of how types of testing complement each other:

• Static code analysis: checking the code without executing it (eg. ESLint)
• Unit tests: checking the code by executing it in isolation (eg. Jasmine)
• End to end: check the code by simulating user interactions, with full in-context code execution (eg. Selenium frameworks like webdriver.io or NightwatchJS)

There are more types of testing, obviously; but this gives some context for where ESLint fits in doing static code analysis. It processes the literal contents of your code against configured rules. The purpose is to provide code quality checks and code style enforcement. It's generally very fast and commonly set up to run whenever files are saved, giving very early error detection.

ESLint is configurable so there's no single, universal rule set (although there are popular options like AirBnB's ruleset). The possible rules are pretty extensive: eslint.org/docs/rules

Common things people check:

• Basic syntax errors
• Declaring vars at the top of scope to prevent hoisting problems
• Code complexity (encourage small, clear units of code that will be easier to unit test)
• Coding style eg. indentation, semicolons, brace style, etc
• Warn if you forget to remove a console log
• Error if there are unused variables (code cruft)

...etc.

So the end result is to lower the error rate in code; and have multiple developers producing code in a similar style (without having to talk about it in code reviews ;)).