Convert javascript object camelCase keys to underscore_case

Question

I want to be able to pass any javascript object containing camelCase keys through a method and return an object with underscore_case keys, mapped to the same values.

So, I have this:

var camelCased = {firstName: 'Jon', lastName: 'Smith'}

And I want a method to output this:

{first_name: 'Jon', last_name: 'Jon'}

What's the fastest way to write a method that takes any object with any number of key/value pairs and outputs the underscore_cased version of that object?

Answer 1

Here's your function to convert camelCase to underscored text (see the jsfiddle):

function camelToUnderscore(key) {
    return key.replace( /([A-Z])/g, "_$1").toLowerCase();
}

console.log(camelToUnderscore('helloWorldWhatsUp'));

Then you can just loop (see the other jsfiddle):

var original = {
    whatsUp: 'you',
    myName: 'is Bob'
},
    newObject = {};

function camelToUnderscore(key) {
    return key.replace( /([A-Z])/g, "_$1" ).toLowerCase();
}

for(var camel in original) {
    newObject[camelToUnderscore(camel)] = original[camel];
}

console.log(newObject);

Answer 2

Marcos Dimitrio posted above with his conversion function, which works but is not a pure function as it changes the original object passed in, which may be an undesireable side effect. Below returns a new object that doesn't modify the original.

export function camelCaseKeysToSnake(obj){
  if (typeof(obj) != "object") return obj;
  let newObj = {...obj}
  for(var oldName in newObj){

      // Camel to underscore
      let newName = oldName.replace(/([A-Z])/g, function($1){return "_"+$1.toLowerCase();});

      // Only process if names are different
      if (newName != oldName) {
          // Check for the old property name to avoid a ReferenceError in strict mode.
          if (newObj.hasOwnProperty(oldName)) {
              newObj[newName] = newObj[oldName];
              delete newObj[oldName];
          }
      }

      // Recursion
      if (typeof(newObj[newName]) == "object") {
          newObj[newName] = camelCaseKeysToSnake(newObj[newName]);
      }
  }
  return newObj;
} 

Answer 3

es6 node solution below. to use, require this file, then pass object you want converted into the function and it will return the camelcased / snakecased copy of the object.

const snakecase = require('lodash.snakecase');

const traverseObj = (obj) => {
  const traverseArr = (arr) => {
    arr.forEach((v) => {
      if (v) {
        if (v.constructor === Object) {
          traverseObj(v);
        } else if (v.constructor === Array) {
          traverseArr(v);
        }
      }
    });
  };

  Object.keys(obj).forEach((k) => {
    if (obj[k]) {
      if (obj[k].constructor === Object) {
        traverseObj(obj[k]);
      } else if (obj[k].constructor === Array) {
        traverseArr(obj[k]);
      }
    }

    const sck = snakecase(k);
    if (sck !== k) {
      obj[sck] = obj[k];
      delete obj[k];
    }
  });
};

module.exports = (o) => {
  if (!o || o.constructor !== Object) return o;

  const obj = Object.assign({}, o);

  traverseObj(obj);

  return obj;
};