typescript/prefer-nullish-coalescing Pedantic

What it does

Enforce using the nullish coalescing operator (??) instead of logical OR (||) or conditional expressions when the left operand might be null or undefined.

Why is this bad?

The || operator returns the right-hand side when the left-hand side is any falsy value (false, 0, '', null, undefined, NaN). This can lead to unexpected behavior when you only want to provide a default for null or undefined.

The nullish coalescing operator (??) only returns the right-hand side when the left-hand side is null or undefined, making the intent clearer and avoiding bugs with other falsy values.

Examples

Examples of incorrect code for this rule:

declare const x: string | null;

// Using || when ?? would be more appropriate
const foo = x || "default";

// Ternary that could use ??
const bar = x !== null && x !== undefined ? x : "default";
const baz = x != null ? x : "default";

// If statement that could use ??
let value = "default";
if (x !== null && x !== undefined) {
  value = x;
}

Examples of correct code for this rule:

declare const x: string | null;

// Using nullish coalescing
const foo = x ?? "default";

// || is fine when you want falsy behavior
declare const y: string;
const bar = y || "default";

// Boolean coercion (can be ignored with ignoreBooleanCoercion)
const bool = Boolean(x || y);

Configuration

This rule accepts a configuration object with the following properties:

allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing

type: boolean

default: false

Unless this is set to true, the rule will error on every file whose tsconfig.json does not have the strictNullChecks compiler option (or strict) set to true.

It is not recommended to enable this config option.

ignoreBooleanCoercion

type: boolean

default: false

Whether to ignore arguments to the Boolean constructor.

ignoreConditionalTests

type: boolean

default: true

Whether to ignore cases that are located within a conditional test.

ignoreIfStatements

type: boolean

default: false

Whether to ignore any if statements that could be simplified by using the nullish coalescing operator.

ignoreMixedLogicalExpressions

type: boolean

default: false

Whether to ignore any logical or expressions that are part of a mixed logical expression (with &&).

ignorePrimitives

type: boolean

Represents the different ways ignorePrimitives can be specified in JSON. Can be:

• true - ignore all primitive types
• An object specifying which primitives to ignore

ignoreTernaryTests

type: boolean

default: false

Whether to ignore any ternary expressions that could be simplified by using the nullish coalescing operator.

How to use

To enable this rule using the config file or in the CLI, you can use:

Config (.oxlintrc.json)

{
  "rules": {
    "typescript/prefer-nullish-coalescing": "error"
  }
}

CLI

oxlint --type-aware --deny typescript/prefer-nullish-coalescing

References

• Rule Source
• Rule Source (tsgolint)