# unicorn/prefer-default-parameters

> Require default parameters instead of reassigning a parameter in the function body

A fallback written as `name = name || "stranger"` is hidden in the function body. A default parameter puts it in the signature, where callers and editors can see it.

The rule reports `||`, `??`, `||=` and `??=` when the fallback is a literal and the parameter is the last one. It reports nothing if an earlier statement in the function could have a side effect, such as a function call.

{/* Generated by src/codegen/lint-rule-docs.ts. Do not edit this section. */}

## Configure

Off by default. To turn it on:

```toml bunfig.toml
[lint.rules]
"unicorn/prefer-default-parameters" = "error" # or "warn"
```

{/* End of the generated section. */}

## Examples

Reported:

{/* bad */}
```js
function greet(name) {
  name = name || "stranger";
  return `Hello, ${name}`;
}
```

{/* bad */}
```js
function connect(host, port) {
  const actualPort = port ?? 8080;
  return open(host, actualPort);
}
```

Not reported:

{/* good */}
```js
function greet(name = "stranger") {
  return `Hello, ${name}`;
}

function connect(host, port = 8080) {
  return open(host, port);
}
```

A parameter that isn't the last one and a fallback that isn't a literal are left alone:

{/* good */}
```js
function connect(host, port) {
  host = host || "localhost";
  port = port || defaultPort();
  return open(host, port);
}
```

## When not to use it

A default parameter only applies to `undefined`. `||` also replaces `0`, `""`, `false` and `null`, and `??` also replaces `null`. If callers pass those values and rely on the fallback, keep the reassignment.