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  3. How to Lowercase Keys of an Object in JavaScript or Node.js

How to Lowercase Keys of an Object in JavaScript or Node.js

When interacting with data from different sources, like your local data and third-party services, the data has different styles and formats. Let’s say you’re using a camelCase format and other sources use everything in UPPERCASE.

Independently of the situation, you may want to transform all keys of an object to lowercase. This tutorial shows you how to do that!

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Lowercase All Keys of an Object in JavaScript

You can use built-in features of JavaScript to lowercase all keys of an object. Modern browsers and Node.js support the Object.entries() method to return an array of the object’s key-value pairs. Then you can iterate over these key-value-pairs using Array#reduce() to create a new object with all keys lowercase.

Here’s a utility function to lowercase all keys in a JavaScript object:

/**
 * Lowercase all top-level keys of the given `object` to lowercase.
 *
 * @returns {Object}
 */
function lowercaseKeys(obj) {  
  if (!isObject(obj)) {
    throw new Error(`You must provide an object to "lowercaseKeys". Received "${typeof obj}"`)
  }

  return Object
    .entries(obj)
    .reduce((carry, [key, value]) => {
      carry[key.toLowerCase()] = value;

      return carry;
    }, {});
}

/**
 * Determine whether the given `value` is an object.
 *
 * @param {*} value
 *
 * @returns {value is object}
 */
function isObject (value) {  
  return Object.prototype.toString.call(value) === '[object Object]'
}

Find more details about the `isObject` helper function in a dedicated tutorial here on Future Studio: [Check If a Value Is an Object in JavaScript or Node.js](https://futurestud.io/tutorials/check-if-a-value-is-an-object-in-javascript-or-node-js)

Please notice: the lowercaseKeys function only lowercases the top-level keys of the object. It’s not recursively going through the object and instead touches the top-level keys only.

Here’s an example of lowercasing all top-level keys of an object:

const fileImport = {  
  fileLocation: '/user/marcus/downloads/data_import.xlsx',
  createdAt: 1610685464300,
  Nested: {
    // the `lowercaseKeys` function does not touch nested objects
  }
}

const lowercasedFileImport = lowercaseKeys(fileImport)  
// {
//   filelocation: '/user/marcus/downloads/data_import.xlsx',
//   createdat: 1610685464300,
//   nested: {
//     … untouched
//   } 
// }

Enjoy lowercasing keys of JavaScript objects!

Marcus Pöhls

Marcus is a fullstack JS developer. He’s passionate about the hapi framework for Node.js and loves to build web apps and APIs. Creator of Futureflix and the “learn hapi” learning path.

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