---
description: 'Enforce using the nullish coalescing operator instead of logical chaining.'
---

> 🛑 This file is source code, not the primary documentation location! 🛑
>
> See **https://typescript-eslint.io/rules/prefer-nullish-coalescing** for documentation.

The `??` nullish coalescing runtime operator allows providing a default value when dealing with `null` or `undefined`.
Because the nullish coalescing operator _only_ coalesces when the original value is `null` or `undefined`, it is much safer than relying upon logical OR operator chaining `||`, which coalesces on any _falsy_ value.

This rule reports when an `||` operator can be safely replaced with a `??`.

:::caution
This rule will not work as expected if [`strictNullChecks`](https://www.typescriptlang.org/tsconfig#strictNullChecks) is not enabled.
:::

## Options

### `ignoreTernaryTests`

Setting this option to `true` (the default) will cause the rule to ignore any ternary expressions that could be simplified by using the nullish coalescing operator.

Incorrect code for `ignoreTernaryTests: false`, and correct code for `ignoreTernaryTests: true`:

```ts
const foo: any = 'bar';
foo !== undefined && foo !== null ? foo : 'a string';
foo === undefined || foo === null ? 'a string' : foo;
foo == undefined ? 'a string' : foo;
foo == null ? 'a string' : foo;

const foo: string | undefined = 'bar';
foo !== undefined ? foo : 'a string';
foo === undefined ? 'a string' : foo;

const foo: string | null = 'bar';
foo !== null ? foo : 'a string';
foo === null ? 'a string' : foo;
```

Correct code for `ignoreTernaryTests: false`:

```ts
const foo: any = 'bar';
foo ?? 'a string';
foo ?? 'a string';
foo ?? 'a string';
foo ?? 'a string';

const foo: string | undefined = 'bar';
foo ?? 'a string';
foo ?? 'a string';

const foo: string | null = 'bar';
foo ?? 'a string';
foo ?? 'a string';
```

### `ignoreConditionalTests`

Setting this option to `true` (the default) will cause the rule to ignore any cases that are located within a conditional test.

Generally expressions within conditional tests intentionally use the falsy fallthrough behavior of the logical or operator, meaning that fixing the operator to the nullish coalesce operator could cause bugs.

If you're looking to enforce stricter conditional tests, you should consider using the `strict-boolean-expressions` rule.

Incorrect code for `ignoreConditionalTests: false`, and correct code for `ignoreConditionalTests: true`:

```ts
declare const a: string | null;
declare const b: string | null;

if (a || b) {
}
while (a || b) {}
do {} while (a || b);
for (let i = 0; a || b; i += 1) {}
a || b ? true : false;
```

Correct code for `ignoreConditionalTests: false`:

```ts
declare const a: string | null;
declare const b: string | null;

if (a ?? b) {
}
while (a ?? b) {}
do {} while (a ?? b);
for (let i = 0; a ?? b; i += 1) {}
a ?? b ? true : false;
```

### `ignoreMixedLogicalExpressions`

Setting this option to `true` (the default) will cause the rule to ignore any logical or expressions that are part of a mixed logical expression (with `&&`).

Generally expressions within mixed logical expressions intentionally use the falsy fallthrough behavior of the logical or operator, meaning that fixing the operator to the nullish coalesce operator could cause bugs.

If you're looking to enforce stricter conditional tests, you should consider using the `strict-boolean-expressions` rule.

Incorrect code for `ignoreMixedLogicalExpressions: false`, and correct code for `ignoreMixedLogicalExpressions: true`:

```ts
declare const a: string | null;
declare const b: string | null;
declare const c: string | null;
declare const d: string | null;

a || (b && c);
(a && b) || c || d;
a || (b && c) || d;
a || (b && c && d);
```

Correct code for `ignoreMixedLogicalExpressions: false`:

```ts
declare const a: string | null;
declare const b: string | null;
declare const c: string | null;
declare const d: string | null;

a ?? (b && c);
(a && b) ?? c ?? d;
a ?? (b && c) ?? d;
a ?? (b && c && d);
```

**_NOTE:_** Errors for this specific case will be presented as suggestions (see below), instead of fixes. This is because it is not always safe to automatically convert `||` to `??` within a mixed logical expression, as we cannot tell the intended precedence of the operator. Note that by design, `??` requires parentheses when used with `&&` or `||` in the same expression.

## When Not To Use It

If you are not using TypeScript 3.7 (or greater), then you will not be able to use this rule, as the operator is not supported.

## Further Reading

- [TypeScript 3.7 Release Notes](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html)
- [Nullish Coalescing Operator Proposal](https://github.com/tc39/proposal-nullish-coalescing/)
