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no-unnecessary-condition

Disallow conditionals where the type is always truthy or always falsy.

🔒

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

🔧

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

💭

This rule requires type information to run.

Any expression being used as a condition must be able to evaluate as truthy or falsy in order to be considered "necessary". Conversely, any expression that always evaluates to truthy or always evaluates to falsy, as determined by the type of the expression, is considered unnecessary and will be flagged by this rule.

The following expressions are checked:

  • Arguments to the &&, || and ?: (ternary) operators
  • Conditions for if, for, while, and do-while statements
  • Base values of optional chain expressions
.eslintrc.cjs
module.exports = {
  "rules": {
    "@typescript-eslint/no-unnecessary-condition": "error"
  }
};
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Examples​

function head<T>(items: T[]) {
  // items can never be nullable, so this is unnecessary
  if (items) {
    return items[0].toUpperCase();
  }
}

function foo(arg: 'bar' | 'baz') {
  // arg is never nullable or empty string, so this is unnecessary
  if (arg) {
  }
}

function bar<T>(arg: string) {
  // arg can never be nullish, so ?. is unnecessary
  return arg?.length;
}

// Checks array predicate return types, where possible
[
  [1, 2],
  [3, 4],
].filter(t => t); // number[] is always truthy
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Options​

This rule accepts the following options

type Options = [
  {
    /** Whether to ignore constant loop conditions, such as `while (true)`. */
    allowConstantLoopConditions?: boolean;
    /** Whether to not error when running with a tsconfig that has strictNullChecks turned. */
    allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing?: boolean;
  },
];

const defaultOptions: Options = [
  {
    allowConstantLoopConditions: false,
    allowRuleToRunWithoutStrictNullChecksIKnowWhatIAmDoing: false,
  },
];

allowConstantLoopConditions​

Example of correct code for { allowConstantLoopConditions: true }:

while (true) {}
for (; true; ) {}
do {} while (true);
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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​

The main downside to using this rule is the need for type information.

This rule has a known edge case of triggering on conditions that were modified within function calls (as side effects). It is due to limitations of TypeScript's type narrowing. See #9998 for details.

We recommend upcasting the variable with a type assertion.

let condition = false as boolean;

const f = () => (condition = true);
f();

if (condition) {
}
  • ESLint: no-constant-condition - no-unnecessary-condition is essentially a stronger version of no-constant-condition, but requires type information.
  • strict-boolean-expressions - a more opinionated version of no-unnecessary-condition. strict-boolean-expressions enforces a specific code style, while no-unnecessary-condition is about correctness.

Resources​