What's the best way (most efficient) to turn all the keys of an object to lower case?

The fastest I come up with is if you create a new object:

var key, keys = Object.keys(obj);
var n = keys.length;
var newobj={}
while (n--) {
  key = keys[n];
  newobj[key.toLowerCase()] = obj[key];
}

I'm not familiar enough with the current inner working of v8 to give you a definitive answer. A few years ago I saw a video where the developers talked about objects, and IIRC it will only delete the references and let the garbage collector take care of it. But it was years ago so even if it was like that then, it doesn't need to be like that now.

Will it bite you later? It depends on what you are doing, but probably not. It is very common to create short lived objects so the code is optimized to handle it. But every environment has its limitations, and maybe it will bite you. You have to test with actual data.

---

Using Object.fromEntries (ES10)

Native and immutable solution using the new Object.fromEntries method:

const newObj = Object.fromEntries(
  Object.entries(obj).map(([k, v]) => [k.toLowerCase(), v])
);

Until that function becomes widely available you could define it yourself with the following polyfill:

Object.fromEntries = arr => Object.assign({}, ...Array.from(arr, ([k, v]) => ({[k]: v}) ));

A nice thing is that this method does the opposite of Object.entries, so now you can go back and forth between the object and array representation.

---

I'd use Lo-Dash.transform like this:

var lowerObj = _.transform(obj, function (result, val, key) {
    result[key.toLowerCase()] = val;
});

---

Personally, I'd use:

let objectKeysToLowerCase = function (origObj) {
    return Object.keys(origObj).reduce(function (newObj, key) {
        let val = origObj[key];
        let newVal = (typeof val === 'object') ? objectKeysToLowerCase(val) : val;
        newObj[key.toLowerCase()] = newVal;
        return newObj;
    }, {});
}

It's succinct, recurs to handle nested objects and returns a new object rather than modifying the original.

In my limited local testing this function is faster than the other recursive solution currently listed (once fixed). I'd love to benchmark it against the others but jsperf is down at the moment (???).

It's also written in ES5.1 so, according to the docs on MDN, should work in FF 4+, Chrome 5+, IE 9.0+, Opera 12+, Safari 5+ (so, pretty much everything).

Object.keys()

Array. prototype.reduce()

Vanilla JS for the win.

I wouldn't worry too much about the garbage collection aspect of all this. Once all references to the old object are destroyed it will be GC's but the new object will still reference basically all it's properties, so they will not.

Any Functions, Arrays or RegExp will be "copied" across by reference. In terms of memory, even Strings will not be duplicated by this process since most (all?) modern JS engines user string interning. I think that leaves just the Numbers, Booleans and the Objects that formed the original structure left to be GC'd.

Note that (all implementations of) this process will lose values if the original has multiple properties with the same lowercase representation. Ie:

let myObj = { xx: 'There', xX: 'can be', Xx: 'only', XX: 'one!' };
console.log(myObj);
// { xx: 'There', xX: 'can be', Xx: 'only', XX: 'one!' }

let newObj = objectKeysToLowerCase(myObj);
console.log(newObj);
// { xx: 'one!' }

Of course, sometimes this is exactly what you want.

Update 2018-07-17

A few people have noted the original function doesn't work well with arrays. Here's an expanded, more resilient version. It recurs correctly through arrays and works if the initial value is an array or simple value:

let objectKeysToLowerCase = function (input) {
    if (typeof input !== 'object') return input;
    if (Array.isArray(input)) return input.map(objectKeysToLowerCase);
    return Object.keys(input).reduce(function (newObj, key) {
        let val = input[key];
        let newVal = (typeof val === 'object') && val !== null ? objectKeysToLowerCase(val) : val;
        newObj[key.toLowerCase()] = newVal;
        return newObj;
    }, {});
};

---

ES6 version:

Object.keys(source)
  .reduce((destination, key) => {
    destination[key.toLowerCase()] = source[key];
    return destination;
  }, {});

---

The loDash/fp way, quite nice as its essentially a one liner

import {
mapKeys
} from 'lodash/fp'

export function lowerCaseObjectKeys (value) {
return mapKeys(k => k.toLowerCase(), value)
}

---

Using forEach seems to be a bit quicker in my tests- and the original reference is gone, so deleting the new one will put it in reach of the g.c.

function keysToLowerCase(obj){
    Object.keys(obj).forEach(function (key) {
        var k = key.toLowerCase();

        if (k !== key) {
            obj[k] = obj[key];
            delete obj[key];
        }
    });
    return (obj);
}

var O={ONE:1,two:2,tHree:3,FOUR:4,Five:5,SIX:{a:1,b:2,c:3,D:4,E:5}}; keysToLowerCase(O);

/* returned value: (Object) */

{
    five:5,
    four:4,
    one:1,
    six:{
        a:1,
        b:2,
        c:3,
        D:4,
        E:5
    },
    three:3,
    two:2
}

---

Simplified Answer

For simple situations, you can use the following example to convert all keys to lower case:

Object.keys(example).forEach(key => {
  const value = example[key];
  delete example[key];
  example[key.toLowerCase()] = value;
});

You can convert all of the keys to upper case using toUpperCase() instead of toLowerCase():

Object.keys(example).forEach(key => {
  const value = example[key];
  delete example[key];
  example[key.toUpperCase()] = value;
});

---

Here is easiest solution to convert all the json keys to lower case.

let o = {"Account_Number   ":"0102301", "customer_NaME":"name"}

o = Object.keys(o).reduce((c, k) => (c[k.toLowerCase().trim()] = o[k], c), {})

console.log(o)

---

I used ES6 and TypeScript. toLowerCaseObject function takes an Array as parameter and looking through Object tree recursively and make every node lowercase:

function toLowerCaseObject(items: any[]) {
        return items.map(x => {
            let lowerCasedObject = {}
                for (let i in x) {
                    if (x.hasOwnProperty(i)) {
                        lowerCased[i.toLowerCase()] = x[i] instanceof Array ? toLowerCaseObject(x[i]) : x[i];
                    }
            }
            return lowerCasedObject;
        });
    }

---

One-liner (only for top level keys):

Object.assign(...Object.keys(obj).map(key => ({[key.toLowerCase()]: obj[key]})))

Converts:

{ a: 1, B: 2, C: { Z: 4 } }

To:

{ a: 1, b: 2, c: { Z: 4 } }

---

With TypeScript

/**
 * Lowercase the keys of an object
 * @example
  lowercaseKeys({FOO: true, bAr: false}); // {foo: true, bar: false}
 */
export function lowercaseKeys<T>(object: { [key: string]: T }): { [key: string]: T } {
  const result: { [key: string]: T } = {};

  for (const [key, value] of Object.entries(object)) {
    result[key.toLowerCase()] = value;
  }

  return result;
}

Usage

lowercaseKeys({FOO: true, bAr: false}); // {foo: true, bar: false}

---

This is not the cleanest way but it has worked for my team so it is worth sharing.

I created this method as our backend is running a language that is not case sensitive and the database and backend will produce different key cases. For us, it has worked flawlessly. Mind you we send dates as Strings and we don't send functions.

We have reduced it to this one line.

const toLowerCase = (data) => JSON.parse(JSON.stringify(data).replace(/"([^"]+)":/g, ($0, key) => '"' + key.toString().toLowerCase() + '":'))

We clone the object by using the JSON.parse(JSON.stringify(obj)) method. This produces a string version of the object in the JSON format. While the object is in the string form you can use regex as JSON is a predictable format to convert all keys.

Broken up it looks like this.

const toLowerCase = function (data) {
  return JSON.parse(JSON.stringify(data)
   .replace(/"([^"]+)":/g, ($0, key) => {
     return '"' + key.toString().toLowerCase() + '":'
   }))
}

---

const keysToLowerCase = object => {
  return Object.keys(object).reduce((acc, key) => {
    let val = object[key];
    if (typeof val === 'object') {
      val = keysToLowerCase(val);
    }
    acc[key.toLowerCase()] = val;
    return acc;
  }, {});
};

Works for nested object.

---

Consider lowering case just once, storing it in a lowKey var:

function keysToLowerCase (obj) {
  var keys = Object.keys(obj);
  var n = keys.length;
  var lowKey;
  while (n--) {
    var key = keys[n];
    if (key === (lowKey = key.toLowerCase()))
    continue

    obj[lowKey] = obj[key]
    delete obj[key]

  }
  return (obj);
}

---

Here's my recursive version based on one of the above examples.

//updated function var lowerObjKeys = function(obj) { Object.keys(obj).forEach(function(key) { var k = key.toLowerCase(); if (k != key) { var v = obj[key] obj[k] = v; delete obj[key]; if (typeof v == 'object') { lowerObjKeys(v); } } }); return obj; } //plumbing console = { _createConsole: function() { var pre = document.createElement('pre'); pre.setAttribute('id', 'console'); document.body.insertBefore(pre, document.body.firstChild); return pre; }, info: function(message) { var pre = document.getElementById("console") || console._createConsole(); pre.textContent += ['>', message, '\n'].join(' '); } }; //test case console.info(JSON.stringify(lowerObjKeys({ "StackOverflow": "blah", "Test": { "LULZ": "MEH" } }), true));

Beware, it doesn't track circular references, so you can end up with an infinite loop resulting in stack overflow.

---

For all values:

to_lower_case = function(obj) {
    for (var k in obj){
        if (typeof obj[k] == "object" && obj[k] !== null)
            to_lower_case(obj[k]);
        else if(typeof obj[k] == "string") {
            obj[k] = obj[k].toLowerCase();
        }
    }
    return obj;
}

Same can be used for keys with minor tweaks.

---

This is how I do it. My input can be anything and it recuses through nested objects as well as arrays of objects.

const fixKeys = input => Array.isArray(input)
  ? input.map(fixKeys)
  : typeof input === 'object'
  ? Object.keys(input).reduce((acc, elem) => {
      acc[elem.toLowerCase()] = fixKeys(input[elem])
      return acc
    }, {})
  : input

tested using mocha

const { expect } = require('chai')

const fixKeys = require('../../../src/utils/fixKeys')

describe('utils/fixKeys', () => {
  const original = {
    Some: 'data',
    With: {
      Nested: 'data'
    },
    And: [
      'an',
      'array',
      'of',
      'strings'
    ],
    AsWellAs: [
      { An: 'array of objects' }
    ]
  }

  const expected = {
    some: 'data',
    with: {
      nested: 'data'
    },
    and: [
      'an',
      'array',
      'of',
      'strings'
    ],
    aswellas: [{ an: 'array of objects' }]
  }

  let result

  before(() => {
    result = fixKeys(original)
  })

  it('left the original untouched', () => {
    expect(original).not.to.deep.equal(expected)
  })

  it('fixed the keys', () => {
    expect(result).to.deep.equal(expected)
  })
})

---

var aa = {ID:1,NAME:'Guvaliour'};
var bb= {};
var cc = Object.keys(aa);
cc.forEach(element=>{
 bb[element.toLowerCase()]=aa[element];
});
cosole.log(bb)

---

The below code to convert the all key in lower case

array.forEach(item=>{
         
          let data = Object.keys(item).reduce((result, p) => (result[p.toLowerCase().trim()] = item[p], result), {})
          
          if(data.hasOwnProperty(fieldname)){
              if(data[fieldname]){
                if(!response['data'].includes(data[fieldname].toLowerCase()))
                    response['data'].push(data[fieldname]) 
                
             }
           }
          
        })

---