Restricts the types allowed in boolean expressions (strict-boolean-expressions)

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.

Examples

Examples of incorrect code for this rule:

// 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();
}

Examples of correct code for this rule:

// Using logical operators for their side effects is allowed
const Component = () => {
  const entry = map.get('foo') || {};
  return entry && <p>Name: {entry.name}</p>;
};

// nullable values should be checked explicitly against null or undefined
let num: number | undefined = 0;
if (num != null) {
  console.log('num is defined');
}

let str: string | null = null;
if (str != null && !str) {
  console.log('str is empty');
}

function foo(bool?: boolean) {
  if (bool ?? false) {
    bar();
  }
}

// `any` types should be cast to boolean explicitly
const foo = (arg: any) => (Boolean(arg) ? 1 : 0);

Options

type Options = {
  allowString?: boolean;
  allowNumber?: boolean;
  allowNullableObject?: boolean;
  allowNullableBoolean?: boolean;
  allowNullableString?: boolean;
  allowNullableNumber?: boolean;
  allowAny?: boolean;
};

const defaultOptions: Options = {
  allowString: true,
  allowNumber: true,
  allowNullableObject: true,
  allowNullableBoolean: false,
  allowNullableString: false,
  allowNullableNumber: 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.

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.

Related To