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.

const 
const toString: (value: any) => string
toString = (
value: any
value) => 
var String: StringConstructor
(value?: any) => string
Allows manipulation and formatting of text strings and determination and location of substrings within strings.
String(
value: any
value);

const 
const toNumber: (value: any) => number
toNumber = function (
value: any
value) {
  return 
var Number: NumberConstructor
(value?: any) => number
An object that represents a number of any kind. All JavaScript numbers are 64-bit floating-point numbers.
Number(
value: any
value);
};

const 
const truthy: unknown[]
truthy = 
const values: unknown[]
values.
Array<unknown>.filter(predicate: (value: unknown, index: number, array: unknown[]) => unknown, thisArg?: any): unknown[] (+1 overload)
Returns the elements of an array that meet the condition specified in a callback function.
@param ― predicate A function that accepts up to three arguments. The filter method calls the predicate function one time for each element in the array.
@param ― thisArg An object to which the this keyword can refer in the predicate function. If thisArg is omitted, undefined is used as the this value.
filter((
value: unknown
value) => 
value: unknown
value);

const 
const hasTruthy: boolean
hasTruthy = 
const values: unknown[]
values.
Array<unknown>.some(predicate: (value: unknown, index: number, array: unknown[]) => unknown, thisArg?: any): boolean
Determines whether the specified callback function returns true for any element of an array.
@param ― predicate A function that accepts up to three arguments. The some method calls
the predicate function for each element in the array until the predicate returns a value
which is coercible to the Boolean value true, or until the end of the array.
@param ― thisArg An object to which the this keyword can refer in the predicate function.
If thisArg is omitted, undefined is used as the this value.
some((
item: unknown
item) => 
item: unknown
item);

const 
const toString: StringConstructor
toString = 
var String: StringConstructor
Allows manipulation and formatting of text strings and determination and location of substrings within strings.
String;

const 
const toNumber: NumberConstructor
toNumber = 
var Number: NumberConstructor
An object that represents a number of any kind. All JavaScript numbers are 64-bit floating-point numbers.
Number;

const 
const truthy: unknown[]
truthy = 
const values: unknown[]
values.
Array<unknown>.filter(predicate: (value: unknown, index: number, array: unknown[]) => unknown, thisArg?: any): unknown[] (+1 overload)
Returns the elements of an array that meet the condition specified in a callback function.
@param ― predicate A function that accepts up to three arguments. The filter method calls the predicate function one time for each element in the array.
@param ― thisArg An object to which the this keyword can refer in the predicate function. If thisArg is omitted, undefined is used as the this value.
filter(
var Boolean: BooleanConstructor
Boolean);

const 
const hasTruthy: boolean
hasTruthy = 
const values: unknown[]
values.
Array<unknown>.some(predicate: (value: unknown, index: number, array: unknown[]) => unknown, thisArg?: any): boolean
Determines whether the specified callback function returns true for any element of an array.
@param ― predicate A function that accepts up to three arguments. The some method calls
the predicate function for each element in the array until the predicate returns a value
which is coercible to the Boolean value true, or until the end of the array.
@param ― thisArg An object to which the this keyword can refer in the predicate function.
If thisArg is omitted, undefined is used as the this value.
some(
var Boolean: BooleanConstructor
Boolean);

const 
const transform: (value: any) => string
transform = (
value: any
value) => 
var String: StringConstructor
(value?: any) => string
Allows manipulation and formatting of text strings and determination and location of substrings within strings.
String(
value: any
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.

Equivalents in Other Linters

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

Made with ❤️‍🔥 around the world by the Flint team and contributors.

builtinCoercions

Examples

Options

When Not To Use It

Further Reading

Equivalents in Other Linters