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I've come up with

function keysToLowerCase (obj) {
  var keys = Object.keys(obj);
  var n = keys.length;
  while (n--) {
    var key = keys[n]; // "cache" it, for less lookups to the array
    if (key !== key.toLowerCase()) { // might already be in its lower case version
        obj[key.toLowerCase()] = obj[key] // swap the value to a new lower case key
        delete obj[key] // delete the old key
    }
  }
  return (obj);
}

But I'm not sure how will v8 behave with that, for instance, will it really delete the other keys or will it only delete references and the garbage collector will bite me later ?

Also, I created these tests, I'm hoping you could add your answer there so we could see how they match up.

EDIT 1: Apparently, according to the tests, it's faster if we don't check if the key is already in lower case, but being faster aside, will it create more clutter by ignoring this and just creating new lower case keys ? Will the garbage collector be happy with this ?

I've come up with

function keysToLowerCase (obj) {
  var keys = Object.keys(obj);
  var n = keys.length;
  while (n--) {
    var key = keys[n]; // "cache" it, for less lookups to the array
    if (key !== key.toLowerCase()) { // might already be in its lower case version
        obj[key.toLowerCase()] = obj[key] // swap the value to a new lower case key
        delete obj[key] // delete the old key
    }
  }
  return (obj);
}

But I'm not sure how will v8 behave with that, for instance, will it really delete the other keys or will it only delete references and the garbage collector will bite me later ?

Also, I created these tests, I'm hoping you could add your answer there so we could see how they match up.

EDIT 1: Apparently, according to the tests, it's faster if we don't check if the key is already in lower case, but being faster aside, will it create more clutter by ignoring this and just creating new lower case keys ? Will the garbage collector be happy with this ?

• javascript
• performance
• node.js
• object
• garbage-collection

Share Improve this question Follow edited Sep 22, 2012 at 0:11 João Pinto Jerónimo asked Sep 22, 2012 at 0:00 João Pinto JerónimoJoão Pinto Jerónimo 9,8541919 gold badges6363 silver badges8787 bronze badges 9

• 2 Can you create a new object rather than modifying the existing one? You would get to skip all the deletes. – Jason Orendorff Commented Sep 22, 2012 at 0:32
• I think your code is reasonable as is. "it's faster if we don't check if the key is already in lower case, but ... will it create more clutter by ignoring this and just creating new lower case keys" - that code doesn't actually work (it will end up deleting any keys that were already lower case), so it really doesn't matter how fast it is... – nnnnnn Commented Sep 22, 2012 at 0:33
• @JasonOrendorff apparently its slower, and it would create one unneeded object every time, and the garbage collector wouldn't be happy with that... – João Pinto Jerónimo Commented Sep 22, 2012 at 0:40
• @JoãoPintoJerónimo What you're seeing in your speed tests is that the code is running thousands of times on the same object. Of course after the test runs once, there's no more work to do; all the keys are lowercase already. If you test it by creating lots of objects with lots of keys: jsperf.com/object-keys-to-lower-case/7 then all three implementations are dramatically slower, but creating a new object is slightly faster in both Firefox and Chrome. – Jason Orendorff Commented Sep 22, 2012 at 3:28
• Oh I see @JasonOrendorff... I'll make another revision of the tests – João Pinto Jerónimo Commented Sep 22, 2012 at 3:34

 |  Show 4 more comments

24 Answers 24

Sorted by: Reset to default 87

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
}

While the ES10 Object.fromentries() method works

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

You can similarly use the snippet below for ES2015 and below

this.htmlWorkbookJSON = jsonData.map((element: Object) => {
    let entriesArray = Object.entries(element)
    const data = new Object()
    entriesArray.forEach(([key, value]) => {
      data[key.toLocaleLowerCase()] = value;
    })
    return data
  })

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;
});

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 } }

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)

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() + '":'
   }))
}

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;
        });
    }

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.

const newObj = {};

for(const key in obj){
    newObj[key.toLowerCase()] = obj[key];
}

const objectToLowercase = (data) => {
      const values = Object.values(data);
      if (values.length === 0) {
        return data;
      }

      return Object.keys(data).reduce((toLowerKeyObj, key) => {
        const isObject = typeof data[key] === 'object';
        const isArray = Array.isArray(data[key]);
        let value = null;

        if (isObject) {
          if (!isArray) {
            value = objectToLowercase(data[key]);
          }
        }

        if (isArray) {
          value = data[key].map(_value => {
            return objectToLowercase(_value);
          });
        }

        toLowerKeyObj[key.toLowerCase()] = isObject ? value : data[key];
        return toLowerKeyObj;
      }, {});
    };

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]) 
                
             }
           }
          
        })

Most of the above answers do not handle null and undefined values. To get around it why not use the transform helper function from lodash?

const query = {
  Company: 'GH Works',
  Items: {
    Construction: 'FB',
    LineItems: {
      Quantity: '100',
      QUALity: 'checked'
    }
  }
}


function deepLowercaseKeys(hash) {
  return _.transform(hash, function(result, value, key) {
    const valueIsObject = typeof value === 'object';
    result[key.toLowerCase()] = valueIsObject ? deepLowercaseKeys(value) : value;
  });
}

console.log(deepLowercaseKeys(query))

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.21/lodash.min.js"></script>

Additionally, you can customize the function and then use it to transform the object in any way you like.

const query = {
  Company: 'GH Works',
  Items: {
    Construction: 'FB',
    LineItems: {
      Quantity: '100',
      QUALity: 'checked'
    }
  }
}

// Base function
function deepTransform(hash, callback) {
  return _.transform(hash, function(result, value, key) {
    if (typeof value === 'object') {
      return callback(result, deepTransform(value, callback), key)
    }
    
    callback(result, value, key)
  })
}

// Custom function (can be anything)
function appendHello(hash) {
  return deepTransform(hash, function(result, value, key) {
    result[`${key}_hello`.toLowerCase()] = value;
  })
}

console.log(appendHello(query))

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.21/lodash.min.js"></script>

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