unicorn/no-instanceof-builtins

instanceof gives the wrong answer for built-ins in two ways. First, a primitive is never an instance, so "text" instanceof String is false. Second, every iframe and every node:vm context has its own built-ins, so an array or a function from another one isn't an instance of your Array or Function.

By default, the rule reports instanceof with Array, Function, String, Number, Boolean, BigInt, and Symbol. --fix-suggestions rewrites these to Array.isArray(value) or a typeof check.

Configure

Off by default. To turn it on:

[lint.rules]
"unicorn/no-instanceof-builtins" = "error" # or "warn"

Every option, set to its default:

[lint.rules]
"unicorn/no-instanceof-builtins" = ["error", {
  strategy = "loose",
  useErrorIsError = false,
  include = [], # list of strings
  exclude = [],
}]

Examples

Reported:

function describe(value) {
  if (value instanceof String) return "text";
  if (value instanceof Function) return "callback";
}

Not reported:

function describe(value) {
  if (typeof value === "string") return "text";
  if (typeof value === "function") return "callback";
  if (value instanceof Map) return "map";
}

Options

Option	Default	What it does
strategy	"loose"	"strict" also reports the error classes, Map, Set, Date, RegExp, Promise, Object, the typed arrays, and other built-in classes.
useErrorIsError	false	Also report instanceof Error, and suggest Error.isError(value).
include	[]	More constructor names to report.
exclude	[]	Constructor names to never report.

if (value instanceof Date || value instanceof TypeError) {
  log(value);
}

if (reason instanceof Error) {
  log(reason.message);
}

if (target instanceof HTMLElement) {
  target.focus();
}