Best Way to Get the Key of a Key/Value JavaScript Object

How to get the key of a key/value JavaScript object

If you want to get all keys, ECMAScript 5 introduced Object.keys. This is only supported by newer browsers but the MDC documentation provides an alternative implementation (which also uses for...in btw):

if(!Object.keys) Object.keys = function(o){
     if (o !== Object(o))
          throw new TypeError('Object.keys called on non-object');
     var ret=[],p;
     for(p in o) if(Object.prototype.hasOwnProperty.call(o,p)) ret.push(p);
     return ret;
}

Of course if you want both, key and value, then for...in is the only reasonable solution.

How to get a key in a JavaScript object by its value?

function getKeyByValue(object, value) {
  return Object.keys(object).find(key => object[key] === value);
}

ES6, no prototype mutations or external libraries.

Example,

function getKeyByValue(object, value) {  return Object.keys(object).find(key => object[key] === value);}

const map = {"first" : "1", "second" : "2"};console.log(getKeyByValue(map,"2"));

How to get a key/value pair for an object

Here you go. To get key/value pair from an object, do it like below using pure JavaScript way Object.entries():

var a = {"e":1,"d":3,"f":4}
for (const [key, value] of Object.entries(a)) {  console.log(`${key} ${value}`);}

Getting key with the highest value from object

For example:

var obj = {a: 1, b: 2, undefined: 1};

Object.keys(obj).reduce(function(a, b){ return obj[a] > obj[b] ? a : b });

In ES6:

var obj = {a: 1, b: 2, undefined: 1};

Object.keys(obj).reduce((a, b) => obj[a] > obj[b] ? a : b);

get key corresponding to value in js object

You can simply fetch all the keys using Object.keys() and then use .find() function to get the key out from that array, and then nicely wrap it in a function to make it modular.

var inspectionMapping = {  'a_pillar_lh': "A Pillar LH",  'b_pillar_lh': "B Pillar LH"};
Object.prototype.getKey = function(value) {  var object = this;  return Object.keys(object).find(key => object[key] === value);};

alert(inspectionMapping.getKey("A Pillar LH"));

how to get the object key if the object and array value matches in javascript

You can simply iterate through the array, and then attempt to fetch the [key, value] tuples returned by Object.entries(obj) whose value matches the array value. Once found, you return the key in the tuple, i.e.:

arr.map(v => Object.entries(obj).find(x => x[1] === v)[0]);

Note: If you array may contain values that are not present in the object, the code above will throw an error because .find() will return undefined. If that's the case, you need to catch cases where an invalid value is used (defensive design):

arr.map(v => {
  const foundTuple = Object.entries(obj).find(x => x[1] === v);
  return foundTuple ? foundTuple[0] : null;
});

See proof-of-concept below:

const obj = {
  "active": 12,
  "inactive": 14,
  "neutral": 16
}

const arr1 = [12];
const arr2 = [12, 14];
const arr3 = [12, 16];
const invalid_arr = [12, 999];

function getKeys(obj, arr) {
  return arr.map(v => {
    const foundTuple = Object.entries(obj).find(x => x[1] === v);
    return foundTuple ? foundTuple[0] : null;
  });
}

console.log(getKeys(obj, arr1));
console.log(getKeys(obj, arr2));
console.log(getKeys(obj, arr3));
console.log(getKeys(obj, invalid_arr));

how to display both key and value in object using javascript?

You can use Object.entries.

let list = {eb: 'blue', et: 'green'}
const keyValue = (input) => Object.entries(input).forEach(([key,value]) => {  console.log(key,value)})

keyValue(list)list = {er: 'yellow', ex: 'black'}keyValue(list)

Swap key with value in object

function swap(json){
  var ret = {};
  for(var key in json){
    ret[json[key]] = key;
  }
  return ret;
}

Example here FIDDLE don't forget to turn on your console to see the results.

ES6 versions:

static objectFlip(obj) {
  const ret = {};
  Object.keys(obj).forEach(key => {
    ret[obj[key]] = key;
  });
  return ret;
}

Or using Array.reduce() & Object.keys()

static objectFlip(obj) {
  return Object.keys(obj).reduce((ret, key) => {
    ret[obj[key]] = key;
    return ret;
  }, {});
}

Or using Array.reduce() & Object.entries()

static objectFlip(obj) {
  return Object.entries(obj).reduce((ret, entry) => {
    const [ key, value ] = entry;
    ret[ value ] = key;
    return ret;
  }, {});
}