# Short-Circuit Evaluation

You've written `if (user && user.name)` without thinking twice about it. But have you ever asked what happens if `user` is `null` - does `user.name` still get evaluated, and crash? It doesn't, and the reason it doesn't is a rule built into nearly every programming language: `&&` and `||` stop evaluating the instant they already know the answer. That behavior has a name - **short-circuit evaluation** - and it's doing more work in your code than you probably realize, for better and for worse.

## How to read this

Read in order. Phase 1 is the rule itself, in its simplest form. Phase 2 shows the two everyday patterns this rule enables - patterns you've almost certainly used already. Phase 3 is the part worth slowing down for: the gotchas that show up when short-circuiting interacts with side effects and falsy values.

## The phases

1. [Why bother checking the second half](01-the-core-rule.md) - the rule: AND stops at the first false, OR stops at the first true.
2. [Where this becomes a real pattern](02-real-patterns.md) - guard checks and default values.
3. [The gotcha](03-the-gotcha.md) - skipped side effects, and the falsy-value surprise with `||`.