A good book I'm reading: Professional JavaScript for Web Developers by Nicholas C. Zakas 3rd Edition has the following information regarding JSON Syntax:

"JSON Syntax allows the representation of three types of values".

Regarding the one you're interested in, Arrays it says:

"Arrays are represented in JSON using array literal notation from JavaScript. For example, this is an array in JavaScript:

var values = [25, "hi", true];

You can represent this same array in JSON using a similar syntax:

[25, "hi", true]

Note the absence of a variable or a semicolon. Arrays and objects can be used together to represent more complex collections of data, such as:

{
    "books":
              [
                {
                    "title": "Professional JavaScript",
                    "authors": [
                        "Nicholas C. Zakas"
                    ],
                    "edition": 3,
                    "year": 2011
                },
                {
                    "title": "Professional JavaScript",
                    "authors": [
                        "Nicholas C.Zakas"
                    ],
                    "edition": 2,
                    "year": 2009
                },
                {
                    "title": "Professional Ajax",
                    "authors": [
                        "Nicholas C. Zakas",
                        "Jeremy McPeak",
                        "Joe Fawcett"
                    ],
                    "edition": 2,
                    "year": 2008
                }
              ]
}

This Array contains a number of objects representing books, Each object has several keys, one of which is "authors", which is another array. Objects and arrays are typically top-level parts of a JSON data structure (even though this is not required) and can be used to create a large number of data structures."

To serialize (convert) a JavaScript object into a JSON string you can use the JSON object stringify() method. For the example from Mark Linus answer:

var cars = [{
    color: 'gray',
    model: '1',
    nOfDoors: 4
    },
    {
    color: 'yellow',
    model: '2',
    nOfDoors: 4
}];

cars is now a JavaScript object. To convert it into a JSON object you could do:

var jsonCars = JSON.stringify(cars);

Which yields:

"[{"color":"gray","model":"1","nOfDoors":4},{"color":"yellow","model":"2","nOfDoors":4}]"

To do the opposite, convert a JSON object into a JavaScript object (this is called parsing), you would use the parse() method. Search for those terms if you need more information... or get the book, it has many examples.

A list of objects is easier to work with. You can use append, map, filter... All the nice things JS Arrays have which manual indexing doesn't. And there's no way to get out of sync, so that's an entire class of bugs gone.

If you're worried about efficiency:

• Measure (premature optimization is the root of all evil)
• Consider the list of lists trick AIWalker proposed
• Consider an outright binary format
• Make sure gzip is enabled
• Measure (it's worth saying twice)

if you are writing all data at once, then you need to do create an array, push all objects to array and write the array to file

function insertDatasJson (res) {
    let fs = require('fs');
    let base = require('../public/json/template.json');
    let result = [];
    for (/*you loop statmeent*/) {
      let obj = JSON.parse(JSON.stringify(base));  // or your preferred way of deep copying
      obj.Subject = 'f';
      obj.Body.Content = 'e';
      obj.Start.DateTime = '2016-11-13T08:30:00';
      obj.End.DateTime = '2016-11-13T17:30:00';
      result.push(obj);
    }

    fs.writeFileSync('./public/json/output/jsonOutput.json', JSON.stringify(result, null, 4));
}

Or if you want to write data in multiple runs, then

function insertDatasJson (res) {
    let fs = require('fs');
    let base = require('../public/json/template.json');
    let data = require('./public/json/output/jsonOutput.json');
    base.Subject = 'f';
    base.Body.Content = 'e';
    base.Start.DateTime = '2016-11-13T08:30:00';
    base.End.DateTime = '2016-11-13T17:30:00';
    data.push(base);
    fs.writeFileSync('./public/json/output/jsonOutput.json', JSON.stringify(data, null, 4));
}

However, in second case, you need to add some code to handle the case of first run when there is no existing data in the output file, or file doesn't exist. Another way to handle that condition would be to initialize the output file with empty JSON array

[]

EDIT: In both cases, appending to the existing file will not work as it will generate invalid JSON.