builtinCoercions

Reports functions that wrap native coercion functions like String, Number, BigInt, Boolean, or Symbol.

✅ This rule is included in the ts stylistic and stylisticStrict presets.

JavaScript’s built-in coercion functions Boolean, BigInt, Number, String, and Symbol can be passed directly where a function is expected. Wrapping them in another function adds unnecessary indirection and reduces code clarity.

This rule also detects identity functions used as callbacks to array filtering methods like .filter(), .some(), .every(), .find(), .findIndex(), .findLast(), and .findLastIndex(). These identity functions are equivalent to passing Boolean directly, which filters out falsy values.

Examples

❌ Incorrect

const toString = (value) => String(value);

const toNumber = function (value) {
  return Number(value);
};

const truthy = values.filter((value) => value);

const hasTruthy = values.some((item) => item);

✅ Correct

const toString = String;

const toNumber = Number;

const truthy = values.filter(Boolean);

const hasTruthy = values.some(Boolean);

const transform = (value) => String(value) + "!";

Options

This rule is not configurable.

When Not To Use It

If you prefer the explicit function wrapper style for consistency with other callbacks that do transform their arguments, you may disable this rule. Some codebases may prefer the visual consistency of always using arrow functions in callbacks, even when a direct function reference would work.

Further Reading

• MDN: String
• MDN: Number
• MDN: Boolean
• MDN: Array.prototype.filter()

Equivalents in Other Linters

• ESLint: unicorn/prefer-native-coercion-functions
• Oxlint: unicorn/prefer-native-coercion-functions