key-of

Returns the first key/index at which the specified value is located in a collection.

It’s like Array.prototype.indexOf() except it works on Maps/Objects/etc.

Installation

Requires Node.js 8.3.0 or above.

npm i key-of

API

The module exports a keyOf() function that has one other function attached to it as a method: keyOf.any().

keyOf()

Parameters

1. Bindable: collection (Array, Iterator, Object, Map, Set, string, or Typed Array)
2. valueToFind (any): The value whose corresponding key or index you want to locate.
3. Optional: Object argument:
   • arrays / maps / sets (arrays of classes/strings): Arrays of classes and/or string names of classes that should be treated as equivalent to Array/Map/Set (respectively).
   • elseReturn (any): A value to return in lieu of the key/index if valueToFind is not found. Only takes effect if no elseThrow is specified. Defaults to undefined.
   • elseThrow (Error or string): An error to be thrown if valueToFind is not found. A string will be wrapped in an Error object automatically.
   • inObj (boolean): Whether or not to search inherited properties if collection is an Object (i.e. not another recognized type). Defaults to false.
   • loose (boolean): Whether or not to identify values loosely (as defined by looselyEquals). Defaults to false.
   • looselyEquals (function): A callback that accepts two values and returns true if they are to be considered equivalent or false otherwise. This argument is only used if loose is true. If omitted, the default behavior will, among other things, consider arrays/objects to be equal if they have the same entries.
   • preferStrict (boolean): Only applies if loose is true. If true, then strictly-identical values will be preferred over loosely-equivalent values. Otherwise, the first loosely-equivalent value found will have its key returned, even if a strictly-identical value comes later. Defaults to false.
   • reflectObj (boolean): Whether or not to use reflection to include non-enumerable Object property values. Only takes effect if collection is an Object (i.e. not another recognized type). Defaults to false.
   • reverse (boolean): Set to true to emulate the behavior of Array.prototype.lastIndexOf(). Defaults to false.

Return Value

• Returns the first key or numeric index (depending on the collection type) at which valueToFind (or its loose equivalent, if so configured) was found in collection.
• If valueToFind is not found in collection, returns elseReturn if set; otherwise undefined.

Example

const keyOf = require('key-of')

const obj = {
  a: 1,
  b: 2,
  c: 2,
  d: {},
  e: {},
}

keyOf(obj, 1) // 'a'
keyOf(obj, 2) // 'b'

keyOf(obj, {}) // undefined
keyOf(obj, {}, {loose: true}) // 'd'

keyOf.any()

Use this function to find the first key (in iteration order) that corresponds to any of the given values. The signature is the same as the main function except that the second parameter is called valuesToFind and takes an iterable (such as an array or string).

Example

const keyOf = require('key-of')

const obj = {
  a: 1,
  b: 2,
  c: 2,
  d: {},
  e: {},
}

keyOf.any(obj, [2, 1]) // 'a'
keyOf.any(obj, [1, {}], {loose: true}) // 'a'

Related

• keys-of: Same as this module, except it returns all keys/indexes, not just the first.