# no-unmodified-loop-condition

> Disallow loop conditions that test variables the loop never changes

If nothing in a `while`, `do...while`, or `for` loop changes the variables in its condition, the loop either never runs or never stops. Usually someone forgot the update, or it changes the wrong variable.

{/* 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]
"no-unmodified-loop-condition" = "error" # or "warn"
```

Every option, set to its default:

```toml bunfig.toml
[lint.rules]
"no-unmodified-loop-condition" = ["error", {
  checkConditionalExpressions = false,
}]
```

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

## Examples

Reported:

{/* bad */}
```js
let node = list.head;

while (node) {
  print(node.value);
  list.next();
}
```

Not reported:

{/* good */}
```js
let node = list.head;

while (node) {
  print(node.value);
  node = node.next;
}
```

In a comparison, the loop only needs to change one side:

{/* good */}
```js
const limit = 3;
let attempts = 0;

while (attempts < limit) {
  attempts++;
}
```

A property or a call's result can change without an assignment, so they aren't checked. Neither is a comparison that contains one:

{/* good */}
```js
let max = 10;

while (queue.length > max) {
  handle(queue);
}
```

A variable also counts as changed if the loop uses a function declaration that assigns to it:

{/* good */}
```js
let done = false;

function finish() {
  done = true;
}

while (!done) {
  finish();
}
```

## Options

| Option | Default | What it does |
| --- | --- | --- |
| `checkConditionalExpressions` | `false` | Check each variable of a `? :` in the condition separately. |

By default a `? :` is treated like a comparison, so one changed variable is enough.

{/* good */}
```js
let urgent = true;
let high = 3;
let low = 5;

while (urgent ? high : low) {
  urgent = isUrgent();
}
```

With the option, `high` and `low` are reported:

{/* bad {"options": [{"checkConditionalExpressions": true}], "count": 2} */}
```js
let urgent = true;
let high = 3;
let low = 5;

while (urgent ? high : low) {
  urgent = isUrgent();
}
```