# Prevents conditionals where the type is always truthy or always falsy (`no-unnecessary-condition`)

Any expression being used as a condition must be able to evaluate as truthy or falsy in order to be considered "necessary". Conversely, any expression that always evaluates to truthy or always evaluates to falsy, as determined by the type of the expression, is considered unnecessary and will be flagged by this rule.

The following expressions are checked:

- Arguments to the `&&`, `||` and `?:` (ternary) operators
- Conditions for `if`, `for`, `while`, and `do-while` statements
- Base values of optional chain expressions

Examples of **incorrect** code for this rule:

```ts
function head<T>(items: T[]) {
  // items can never be nullable, so this is unnecessary
  if (items) {
    return items[0].toUpperCase();
  }
}

function foo(arg: 'bar' | 'baz') {
  // arg is never nullable or empty string, so this is unnecessary
  if (arg) {
  }
}

function bar<T>(arg: string) {
  // arg can never be nullish, so ?. is unnecessary
  return arg?.length;
}

// Checks array predicate return types, where possible
[
  [1, 2],
  [3, 4],
].filter(t => t); // number[] is always truthy
```

Examples of **correct** code for this rule:

```ts
function head<T>(items: T[]) {
  // Necessary, since items.length might be 0
  if (items.length) {
    return items[0].toUpperCase();
  }
}

function foo(arg: string) {
  // Necessary, since foo might be ''.
  if (arg) {
  }
}

function bar(arg?: string | null) {
  // Necessary, since arg might be nullish
  return arg?.length;
}

[0, 1, 2, 3].filter(t => t); // number can be truthy or falsy
```

## Options

```ts
type Options = {
  // if true, the rule will ignore constant loop conditions
  allowConstantLoopConditions?: boolean;
  // if true, the rule will not error when running with a tsconfig that has strictNullChecks turned **off**
  allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing?: boolean;
};

const defaultOptions: Options = {
  allowConstantLoopConditions: false,
  allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing: false,
};
```

### `allowConstantLoopConditions`

Example of correct code for `{ allowConstantLoopConditions: true }`:

```ts
while (true) {}
for (; true; ) {}
do {} while (true);
```

### `allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing`

If this is set to `false`, then the rule will error on every file whose `tsconfig.json` does _not_ have the `strictNullChecks` compiler option (or `strict`) set to `true`.

Without `strictNullChecks`, TypeScript essentially erases `undefined` and `null` from the types. This means when this rule inspects the types from a variable, **it will not be able to tell that the variable might be `null` or `undefined`**, which essentially makes this rule useless.

You should be using `strictNullChecks` to ensure complete type-safety in your codebase.

If for some reason you cannot turn on `strictNullChecks`, but still want to use this rule - you can use this option to allow it - but know that the behavior of this rule is _undefined_ with the compiler option turned off. We will not accept bug reports if you are using this option.

## When Not To Use It

The main downside to using this rule is the need for type information.

## Related To

- ESLint: [no-constant-condition](https://eslint.org/docs/rules/no-constant-condition) - `no-unnecessary-condition` is essentially a stronger version of `no-constant-condition`, but requires type information.

- [strict-boolean-expressions](./strict-boolean-expressions.md) - a more opinionated version of `no-unnecessary-condition`. `strict-boolean-expressions` enforces a specific code style, while `no-unnecessary-condition` is about correctness.