prefer-nullish-coalescing

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

🎨

Extending "plugin:@typescript-eslint/stylistic-type-checked" in an ESLint configuration enables this rule.

💡

Some problems reported by this rule are manually fixable by editor suggestions.

💭

This rule requires type information to run.

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 you may consider replacing:

• An || operator with ??
• An ||= operator with ??=

caution

This rule will not work as expected if strictNullChecks is not enabled.

Flat Config

eslint.config.mjs

export default tseslint.config({
  rules: {
    "@typescript-eslint/prefer-nullish-coalescing": "error"
  }
});

Legacy Config

.eslintrc.cjs

module.exports = {
  "rules": {
    "@typescript-eslint/prefer-nullish-coalescing": "error"
  }
};

Try this rule in the playground ↗

Options

This rule accepts the following options:

type Options = [
  {
    /** 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`. */
    allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing?: boolean;
    /** Whether to ignore arguments to the `Boolean` constructor */
    ignoreBooleanCoercion?: boolean;
    /** Whether to ignore cases that are located within a conditional test. */
    ignoreConditionalTests?: boolean;
    /** Whether to ignore any logical or expressions that are part of a mixed logical expression (with `&&`). */
    ignoreMixedLogicalExpressions?: boolean;
    /** Whether to ignore all (`true`) or some (an object with properties) primitive types. */
    ignorePrimitives?: /**
     * Whether to ignore all (`true`) or some (an object with properties) primitive types.
     * Which primitives types may be ignored.
     */
    | {
          /** Ignore bigint primitive types. */
          bigint?: boolean;
          /** Ignore boolean primitive types. */
          boolean?: boolean;
          /** Ignore number primitive types. */
          number?: boolean;
          /** Ignore string primitive types. */
          string?: boolean;
          [k: string]: unknown;
        }
      /** Ignore all primitive types. */
      | true;
    /** Whether to ignore any ternary expressions that could be simplified by using the nullish coalescing operator. */
    ignoreTernaryTests?: boolean;
  },
];

const defaultOptions: Options = [
  {
    allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing: false,
    ignoreBooleanCoercion: false,
    ignoreConditionalTests: true,
    ignoreMixedLogicalExpressions: false,
    ignorePrimitives: {
      bigint: false,
      boolean: false,
      number: false,
      string: false,
    },
    ignoreTernaryTests: false,
  },
];

ignoreTernaryTests

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

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

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;

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Correct code for ignoreTernaryTests: false:

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';

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ignoreConditionalTests

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

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:

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

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

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Correct code for ignoreConditionalTests: false:

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

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

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ignoreMixedLogicalExpressions

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

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:

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

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Correct code for ignoreMixedLogicalExpressions: false:

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

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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.

ignorePrimitives

Whether to ignore all (true) or some (an object with properties) primitive types. Default: {"bigint":false,"boolean":false,"number":false,"string":false}.

If you would like to ignore expressions containing operands of certain primitive types that can be falsy then you may pass an object containing a boolean value for each primitive:

• string: true, ignores null or undefined unions with string (default: false).
• number: true, ignores null or undefined unions with number (default: false).
• bigint: true, ignores null or undefined unions with bigint (default: false).
• boolean: true, ignores null or undefined unions with boolean (default: false).

Incorrect code for ignorePrimitives: { string: false }, and correct code for ignorePrimitives: { string: true }:

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

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Correct code for both ignorePrimitives: { string: false } and ignorePrimitives: { string: true }:

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

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Also, if you would like to ignore all primitives types, you can set ignorePrimitives: true. It is equivalent to ignorePrimitives: { string: true, number: true, bigint: true, boolean: true }.

ignoreBooleanCoercion

Whether to ignore arguments to the Boolean constructor Default: false.

Whether to ignore expressions that coerce a value into a boolean: Boolean(...).

Incorrect code for ignoreBooleanCoercion: false, and correct code for ignoreBooleanCoercion: true:

let a: string | true | undefined;
let b: string | boolean | undefined;

const x = Boolean(a || b);

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Correct code for ignoreBooleanCoercion: false:

let a: string | true | undefined;
let b: string | boolean | undefined;

const x = Boolean(a ?? b);

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allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing

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. Default: false.

Deprecated

This option will be removed in the next major version of typescript-eslint. ::: 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.

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

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
• Nullish Coalescing Operator Proposal

When Not To Use It

Type checked lint rules are more powerful than traditional lint rules, but also require configuring type checked linting.

See Troubleshooting > Linting with Type Information > Performance if you experience performance degradations after enabling type checked rules.

Resources

• Rule source
• Test source