#prefer-nullish-coalescing

#Configuration

import { defineConfig, ts } from '@rslint/core';

export default defineConfig([
  ts.configs.recommended,
  {
    rules: {
      '@typescript-eslint/prefer-nullish-coalescing': 'error',
    },
  },
]);

Enforce using the nullish coalescing operator instead of logical assignments or chaining.

#Rule Details

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 ||, ||=, conditional expressions, and if-statement assignments could be replaced with the nullish-coalescing operator.

This rule requires strictNullChecks to be enabled in tsconfig.json to function correctly.

Examples of incorrect code for this rule:

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

a || b;
a || 'fallback';
a ||= b;

a !== undefined && a !== null ? a : 'a string';
a === undefined || a === null ? 'a string' : a;

declare let foo: { a: string } | null;
declare function makeFoo(): { a: string };
if (!foo) {
  foo = makeFoo();
}

Examples of correct code for this rule:

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

a ?? b;
a ?? 'fallback';
a ??= b;

declare let foo: { a: string } | null;
declare function makeFoo(): { a: string };
foo ??= makeFoo();

#Options

#ignoreConditionalTests

Default: true. When true, ignore cases that appear inside the test of if/while/do…while/for loops or in the test of a conditional expression.

{ "@typescript-eslint/prefer-nullish-coalescing": ["error", { "ignoreConditionalTests": false }] }

#ignoreTernaryTests

Default: false. When true, ignore ternary expressions that could be replaced with ??.

#ignoreIfStatements

Default: false. When true, ignore if statements that could be replaced with ??=.

#ignoreMixedLogicalExpressions

Default: false. When true, ignore || expressions that are part of a mixed logical expression (with &&).

#ignoreBooleanCoercion

Default: false. When true, ignore || arguments to the global Boolean constructor.

#ignorePrimitives

Default: { bigint: false, boolean: false, number: false, string: false }. Set to true to ignore all listed primitives, or to a partial object to ignore individual primitive types.

{ "@typescript-eslint/prefer-nullish-coalescing": ["error", { "ignorePrimitives": { "string": true } }] }

#allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing

Default: false. By default the rule errors on every file when strictNullChecks is off. Setting this to true silences that error and lets the rule run anyway.

#Original Documentation

@typescript-eslint/prefer-nullish-coalescing