util.js

// Although `Symbol` is widely supported these days, we can safely fall
// back to using a non-enumerable string property without violating any
// assumptions elsewhere in the implementation.
const useSymbol =
  typeof Symbol === "function" &&
  typeof Symbol.for === "function";

// Used to mark `tuple.prototype` so that all objects that inherit from
// any `tuple.prototype` object (there could be more than one) will test
// positive according to `tuple.isTuple`.
export const brand = useSymbol
  ? Symbol.for("immutable-tuple")
  : "@@__IMMUTABLE_TUPLE__@@";

// Used to save a reference to the globally shared `UniversalWeakMap` that
// stores all known `tuple` objects.
export const globalKey = useSymbol
  ? Symbol.for("immutable-tuple-root")
  : "@@__IMMUTABLE_TUPLE_ROOT__@@";

// Convenient helper for defining hidden immutable properties.
export function def(obj, name, value, enumerable) {
  Object.defineProperty(obj, name, {
    value: value,
    enumerable: !! enumerable,
    writable: false,
    configurable: false
  });
  return value;
}

export const freeze = Object.freeze || function (obj) {
  return obj;
};

export function isObjRef(value) {
  switch (typeof value) {
  case "object":
    if (value === null) {
      return false;
    }
  case "function":
    return true;
  default:
    return false;
  }
}

// The `mustConvertThisToArray` value is true when the corresponding
// `Array` method does not attempt to modify `this`, which means we can
// pass a `tuple` object as `this` without first converting it to an
// `Array`.
export function forEachArrayMethod(fn) {
  function call(name, mustConvertThisToArray) {
    const desc = Object.getOwnPropertyDescriptor(Array.prototype, name);
    fn(name, desc, !! mustConvertThisToArray);
  }

  call("every");
  call("filter");
  call("find");
  call("findIndex");
  call("forEach");
  call("includes");
  call("indexOf");
  call("join");
  call("lastIndexOf");
  call("map");
  call("reduce");
  call("reduceRight");
  call("slice");
  call("some");
  call("toLocaleString");
  call("toString");

  // The `reverse` and `sort` methods are usually destructive, but for
  // `tuple` objects they return a new `tuple` object that has been
  // appropriately reversed/sorted.
  call("reverse", true);
  call("sort", true);

  // Make `[...someTuple]` work.
  call(useSymbol && Symbol.iterator || "@@iterator");
}