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strict-boolean-expressions

Disallow certain types in boolean expressions.

🔧

Some problems reported by this rule are automatically fixable by the --fix ESLint command line option.

💡

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

💭

This rule requires type information to run.

Forbids usage of non-boolean types in expressions where a boolean is expected. boolean and never types are always allowed. Additional types which are considered safe in a boolean context can be configured via options.

The following nodes are considered boolean expressions and their type is checked:

  • Argument to the logical negation operator (!arg).
  • The condition in a conditional expression (cond ? x : y).
  • Conditions for if, for, while, and do-while statements.
  • Operands of logical binary operators (lhs || rhs and lhs && rhs).
    • Right-hand side operand is ignored when it's not a descendant of another boolean expression. This is to allow usage of boolean operators for their short-circuiting behavior.
.eslintrc.cjs
module.exports = {
  "rules": {
    "@typescript-eslint/strict-boolean-expressions": "error"
  }
};

Try this rule in the playground ↗

Examples​

// nullable numbers are considered unsafe by default
let num: number | undefined = 0;
if (num) {
  console.log('num is defined');
}

// nullable strings are considered unsafe by default
let str: string | null = null;
if (!str) {
  console.log('str is empty');
}

// nullable booleans are considered unsafe by default
function foo(bool?: boolean) {
  if (bool) {
    bar();
  }
}

// `any`, unconstrained generics and unions of more than one primitive type are disallowed
const foo = <T>(arg: T) => (arg ? 1 : 0);

// always-truthy and always-falsy types are disallowed
let obj = {};
while (obj) {
  obj = getObj();
}
Open in Playground

Options​

This rule accepts the following options:

type Options = [
  {
    allowAny?: boolean;
    allowNullableBoolean?: boolean;
    allowNullableEnum?: boolean;
    allowNullableNumber?: boolean;
    allowNullableObject?: boolean;
    allowNullableString?: boolean;
    allowNumber?: boolean;
    allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing?: boolean;
    allowString?: boolean;
  },
];

const defaultOptions: Options = [
  {
    allowString: true,
    allowNumber: true,
    allowNullableObject: true,
    allowNullableBoolean: false,
    allowNullableString: false,
    allowNullableNumber: false,
    allowNullableEnum: false,
    allowAny: false,
    allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing: false,
  },
];

allowString​

Allows string in a boolean context. This is safe because strings have only one falsy value (""). Set this to false if you prefer the explicit str != "" or str.length > 0 style.

allowNumber​

Allows number in a boolean context. This is safe because numbers have only two falsy values (0 and NaN). Set this to false if you prefer the explicit num != 0 and !Number.isNaN(num) style.

allowNullableObject​

Allows object | function | symbol | null | undefined in a boolean context. This is safe because objects, functions and symbols don't have falsy values. Set this to false if you prefer the explicit obj != null style.

allowNullableBoolean​

Allows boolean | null | undefined in a boolean context. This is unsafe because nullable booleans can be either false or nullish. Set this to false if you want to enforce explicit bool ?? false or bool ?? true style. Set this to true if you don't mind implicitly treating false the same as a nullish value.

allowNullableString​

Allows string | null | undefined in a boolean context. This is unsafe because nullable strings can be either an empty string or nullish. Set this to true if you don't mind implicitly treating an empty string the same as a nullish value.

allowNullableNumber​

Allows number | null | undefined in a boolean context. This is unsafe because nullable numbers can be either a falsy number or nullish. Set this to true if you don't mind implicitly treating zero or NaN the same as a nullish value.

allowNullableEnum​

Allows enum | null | undefined in a boolean context. This is unsafe because nullable enums can be either a falsy number or nullish. Set this to true if you don't mind implicitly treating an enum whose value is zero the same as a nullish value.

allowAny​

Allows any in a boolean context. This is unsafe for obvious reasons. Set this to true at your own risk.

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 a lot less useful.

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.

Fixes and Suggestions​

This rule provides following fixes and suggestions for particular types in boolean context:

  • boolean - Always allowed - no fix needed.
  • string - (when allowString is false) - Provides following suggestions:
    • Change condition to check string's length (str → str.length > 0)
    • Change condition to check for empty string (str → str !== "")
    • Explicitly cast value to a boolean (str → Boolean(str))
  • number - (when allowNumber is false):
    • For array.length - Provides autofix:
      • Change condition to check for 0 (array.length → array.length > 0)
    • For other number values - Provides following suggestions:
      • Change condition to check for 0 (num → num !== 0)
      • Change condition to check for NaN (num → !Number.isNaN(num))
      • Explicitly cast value to a boolean (num → Boolean(num))
  • object | null | undefined - (when allowNullableObject is false) - Provides autofix:
    • Change condition to check for null/undefined (maybeObj → maybeObj != null)
  • boolean | null | undefined - Provides following suggestions:
    • Explicitly treat nullish value the same as false (maybeBool → maybeBool ?? false)
    • Change condition to check for true/false (maybeBool → maybeBool === true)
  • string | null | undefined - Provides following suggestions:
    • Change condition to check for null/undefined (maybeStr → maybeStr != null)
    • Explicitly treat nullish value the same as an empty string (maybeStr → maybeStr ?? "")
    • Explicitly cast value to a boolean (maybeStr → Boolean(maybeStr))
  • number | null | undefined - Provides following suggestions:
    • Change condition to check for null/undefined (maybeNum → maybeNum != null)
    • Explicitly treat nullish value the same as 0 (maybeNum → maybeNum ?? 0)
    • Explicitly cast value to a boolean (maybeNum → Boolean(maybeNum))
  • any and unknown - Provides following suggestions:
    • Explicitly cast value to a boolean (value → Boolean(value))

When Not To Use It​

If your project isn't likely to experience bugs from falsy non-boolean values being used in logical conditions, you can skip enabling this rule.

Otherwise, this rule can be quite strict around requiring exact comparisons in logical checks. If you prefer more succinct checks over more precise boolean logic, this rule might not be for you.

Type checked lint rules are more powerful than traditional lint rules, but also require configuring type checked linting. See Performance Troubleshooting if you experience performance degredations after enabling type checked rules.

Resources​