JSON Manipulation with Javascript

{“quote”:”You miss 100% of the shots you don’t take.”,”author”:”Wayne Gretzky”}


JavaScript Object Notation (JSON) is a standard text-based format for representing structured data based on JavaScript object syntax. Extension is .json, and MIME type is application/json.

It’s broadly used in data transaction between application and servers.

JS only has one data type which is Object. Array is a special form of Object.

  • Plain object: {key0:value, key1:value, ...}
  • Array: [value,value, ...]

Both objects and arrays expose a key -> value structure. In objects, it’s obvious. In Arrays, the keys are numeric index. Values in Objects can be reached by its keys. Similarly, values in Arrays can be gotten by its index.

Object and string conversion

In JS, JSON is an object, however, in a transaction, JSON is a string. JS provides methods in JSON object to convert between these two.

  • From String to JSON object

        person = JSON.parse(data);
    const jsonObject = JSON.parse(jsonString);
    const str = JSON.stringify(arr);

Loop through JSON Objects

  • First option is to loop through all key -> value pages,
    for (const prop in data) { }
  • The second option is to get all keys and loop through these keys to reach each key -> value pair
    Object.keys(data).forEach(function(prop) {
      // `prop` is the property name
      // `data[prop]` is the property value

Loop through JSON Arrays

  • First, here is how we use for ( ; ; )loop
      for(let i = 0, l = data.persons.length; i < l; i++) {
          // var person = data.persons[i];
          // `person.id` and `person.name`. 
          // We could also use `data.persons[i].id`.
  • Second way to loop is to use for... in. It’s not a good practice because:
    • First, the order of the loop is undefined for a for...in loop, so there’s no guarantee the properties will be iterated in the order you want.
    • Second, for…in iterates over all enumerable properties of an object, including those inherited from its prototype.You can use Object.prototype.hasOwnProperty() to check that the property is owned directly by the object rather than being inherited.
    • If you really need to use this way, you need to remove additional augment in the prototype of Array.
    Array.prototype.remove = function(val) {
        // Irrelevant implementation details
    var a = ["a", "b", "c"];
    for (var i in a) {
    //0,1,2, which are index of elements  
  • Thirdly, in ES5, we can use forEach method
    data.items.forEach(function(value, index, array) {
        // The callback is executed for each element in the array.
        // `value` is the element itself (equivalent to `array[index]`)
        // `index` will be the index of the element in the array
        // `array` is a reference to the array itself (i.e. `data.items` in this case)
  • Fourthly, using for...of method.

    let iterable = [10, 20, 30];
    for (let value of iterable) {
      value += 1;
    // 11
    // 21
    // 31

Read a JSON tree recursively

// This function handles arrays and objects
function eachRecursive(obj)
    for (var k in obj)
        if (typeof obj[k] == "object" && obj[k] !== null)
            // do something... 

Get keys, values of an object

  • Object.keys() returns an array of object’s keys,
  • Object.values() returns an array of object’s values,
  • Object.entries() returns an array of object’s keys and corresponding values in a format [key, value].
const obj = { a: 1 ,b: 2 ,c: 3 }

console.log(Object.keys(obj)) // ['a', 'b', 'c']
console.log(Object.values(obj)) // [1, 2, 3]
console.log(Object.entries(obj)) // [['a', 1], ['b', 2], ['c', 3]]

//for-of loop to loop through enties
for (const [key, value] of Object.entries(obj)) {
console.log(`key: ${key}, value: ${value}`)

//since it's an Array inside of the entiry
for (const element of Object.entries(obj)) {
  const key   = element[0]
  const value = element[1]

Convert a http response to object

this.restService.post('/projects', true, JSON.stringify(project))
    .subscribe((response: Project) => {
            const newProject: Project = new Project();

Merge two jsons, or say values overwriting

Object.assign(target, ...sources)

var o1 = { a: 1, b: 1, c: 1 };
var o2 = { b: 2, c: 2 };
var o3 = { c: 3 };

var obj = Object.assign({}, o1, o2, o3);
console.log(obj); // { a: 1, b: 2, c: 3 }