Efficiently rename/re-map javascript/json object keys within array of objects

Question 1

As I already mentioned in the comments, if you can make certain assumptions about the values of the objects, you could use a regular expression to replace the keys, for example:

str = str.replace(/"title":/g, '"name":');

It's not as "clean", but it might get the job done faster.

If you have to parse the JSON anyway, a more structured approach would be to pass a reviver function to JSON.parse and you might be able to avoid an additional pass over the array. This probably depends on how engine implement JSON.parse though (maybe they parse the whole string first and then make a second pass with the reviver function, in which case you wouldn't get any advantage).

var arr = JSON.parse(str, function(prop, value) {
   switch(prop) {
     case "title":
        this.name = value;
        return;
     case "uid":
        this.id = value;
        return;
     default:
        return value;
   }
});

Benchmarks, using the Node.js script below to test 3 times:

1389822740739: Beginning regex rename test
1389822740761: Regex rename complete
// 22ms, 22ms, 21ms
1389822740762: Beginning parse and remap in for loop test
1389822740831: For loop remap complete
// 69ms, 68ms, 68ms
1389822740831: Beginning reviver function test
1389822740893: Reviver function complete
// 62ms, 61ms, 60ms

It appears as if the regex (in this case) is the most efficient, but be careful when trying to parse JSON with regular expressions.

Test script, loading 100,230 lines of the OP's sample JSON:

fs = require('fs');
fs.readFile('test.json', 'utf8', function (err, data) {
    if (err) {
        return console.log(err);
    }
    console.log(new Date().getTime() + ": Beginning regex rename test");
    var str = data.replace(/"title":/g, '"name":');
    str = str.replace(/"uid":/g, '"id":');
    JSON.parse(str);
    console.log(new Date().getTime() + ": Regex rename complete");
    console.log(new Date().getTime() + ": Beginning parse and remap in for loop test");
    var arr = JSON.parse(data);
    for (var i = 0; i < arr.length; i++) {
        arr[i].name = arr[i].title;
        arr[i].id = arr[i].uid;
        delete arr[i].title;
        delete arr[i].uid;
    }
    console.log(new Date().getTime() + ": For loop remap complete");
    console.log(new Date().getTime() + ": Beginning reviver function test");
    var arr = JSON.parse(data, function (prop, value) {
        switch (prop) {
            case "title":
                this.name = value;
                return;
            case "uid":
                this.id = value;
                return;
            default:
                return value;
        }
    });
    console.log(new Date().getTime() + ": Reviver function complete");
});

Question 2

Asked this question a long time ago, and since then, I've grown acustomed to using Array.prototype.map() to get the job done, more for stability and cleanliness of code than performance. While it's certainly not the most performant, it looks great:

var repl = orig.map(function(obj) {
    return {
        name: obj.title,
        id: obj.uid
    }
})

If you need a more flexible (and ES6-compatible function), try:

let replaceKeyInObjectArray = (a, r) => a.map(o => 
    Object.keys(o).map((key) => ({ [r[key] || key] : o[key] })
).reduce((a, b) => Object.assign({}, a, b)))

e.g.

const arr = [{ abc: 1, def: 40, xyz: 50 }, { abc: 1, def: 40, xyz: 50 }, { abc: 1, def: 40, xyz: 50 }]
const replaceMap = { "abc": "yyj" }

replaceKeyInObjectArray(arr, replaceMap)

/*
[
    {
        "yyj": 1,
        "def": 40,
        "xyz": 50
    },
    {
        "yyj": 1,
        "def": 40,
        "xyz": 50
    },
    {
        "yyj": 1,
        "def": 40,
        "xyz": 50
    }
]
*/

Question 3

Here's another take on the OP's suggestion to use map() for clarity (not performance).

var newItems = items.map(item => ({
    name: item.title,
    id: item.uid
}));

This uses ES6 arrow functions and the shortcut syntaxes that are possible when there's only one parm passed to the function and only one statement in the body of the function.

Depending on your history with lambda expressions in various languages, this form may or may not resonate with you.

Be careful when returning an object literal in the arrow function shortcut syntax like this. Don't forget the additional parens around the object literal!

Question 4

If you want to make it a little more reusable. Maybe this is a decent approach.

function rekey(arr, lookup) {
	for (var i = 0; i < arr.length; i++) {
		var obj = arr[i];
		for (var fromKey in lookup) {
			var toKey = lookup[fromKey];
			var value = obj[fromKey];
			if (value) {
				obj[toKey] = value;
				delete obj[fromKey];
			}
		}
	}
	return arr;
}

var arr = [{ apple: 'bar' }, { apple: 'foo' }];
var converted = rekey(arr, { apple: 'kung' });
console.log(converted);

Question 5

Using ES6:

const renameFieldInArrayOfObjects = (arr, oldField, newField) => {
  return arr.map(s => {
    return Object.keys(s).reduce((prev, next) => {
      if(next === oldField) { 
        prev[newField] = s[next]
      } else { 
        prev[next] = s[next] 
      }
      return prev
    }, {})
  })
}

Using ES7:

const renameFieldInArrayOfObjects = (arr, oldField, newField) => {
  return arr.map(s => {
    return Object.keys(s).reduce((prev, next) => {
      return next === oldField
        ? {...prev, [newField]: s[next]}
        : {...prev, [next]: s[next]}
    }, {})
  })
}

Question 6

You can use an npm package named node-data-transform.

Your data :

const data = [
  {
    title: 'pineapple',
    uid: 'ab982d34c98f',
  },
  {
    title: 'carrots',
    uid: '6f12e6ba45ec',
  },
];

Your mapping :

const map = {
  item: {
    name: 'title',
    id: 'uid',
  },
};

And use the package :

const DataTransform = require("node-json-transform").DataTransform;
const dataTransform = DataTransform(data, map);
const result = dataTransform.transform();
console.log(result);

Result :

[
  {
    name: 'pineapple',
    id: 'ab982d34c98f'
  },
  {
    name: 'carrots',
    id: '6f12e6ba45ec'
  }
]

Maybe it's not the best way for performance, but it's quite elegant.

Question 7

var jsonObj = [/*sample array in question*/   ]

Based on different benchmarks discussed below, fastest solution is native for:

var arr = [];
for(var i = 0, len = jsonObj .length; i < len; i++) {
  arr.push( {"name": jsonObj[i].title, "id" : jsonObj[i].uid});
}

I think alternatively without using a frameworks this will be option 2:

var arr = []
jsonObj.forEach(function(item) { arr.push({"name": item.title, "id" : item.uid }); });

There is always debate between using navite and non-navite functions. If I remember correctly lodash argued they were faster than underscore because the use non-native functions for key operations.

However different browsers will produce sometimes very different results. I always looked for the best average.

For benchmarks you can take a look at this:

http://jsperf.com/lo-dash-v1-1-1-vs-underscore-v1-4-4/8

Question 8

function replaceElem(value, replace, str) {
            while (str.indexOf(value) > -1) {
                str = str.replace(value, replace);
            }
            return str;
        }

call this from main

var value = "tittle";
var replace = "name";
replaceElem(value, replace, str);