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.

.eslintrc.cjs

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

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Options

This rule accepts the following options:

type Options = [
  {
    allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing?: boolean;
    ignoreConditionalTests?: boolean;
    ignoreMixedLogicalExpressions?: boolean;
    ignorePrimitives?:
      | {
          bigint?: boolean;
          boolean?: boolean;
          number?: boolean;
          string?: boolean;
          [k: string]: unknown;
        }
      | true;
    ignoreTernaryTests?: boolean;
  },
];

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

ignoreTernaryTests

Setting this option to true will cause the rule to ignore any ternary expressions that could be simplified by using the nullish coalescing operator. This is set to false by default.

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

Setting this option to false will cause the rule to also check cases that are located within a conditional test. This is set to true by default.

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

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

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

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 }.

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

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

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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 degredations after enabling type checked rules.

Resources

• Rule source
• Test source