Simple:

var data = [{"name":"Marine Lines","location_id":3},{"name":"Ghatkopar","location_id":2}]
var result = data.map(function(val) {
  return val.location_id;
}).join(',');

console.log(result)

I assume you wanted a string, hence the .join(','), if you want an array simply remove that part.

This can be achieved via the join() method which is built into the Array type:

const object = {
"id" : "122223232244",
"title" : "התרעת פיקוד העורף",
"data" : ["עוטף עזה 218","עוטף עזה 217"]
}

/* Join elements of data array in object to a comma separated string */
const value = object.data.join();

console.log(value);

If no separator argument is supplied, then the join() method will default to use a comma separator by default.

Update

If the JSON was supplied in raw text via a string you can use the JSON.parse() method to extract an object from the JSON string value as a first step like so:

const json = `{"id" : "122223232244","title" : "התרעת פיקוד העורף","data" : ["עוטף עזה 218","עוטף עזה 217"]}`

/* Parse input JSON string */
const object = JSON.parse(json);

/* Join elements of data array in object to a comma separated string */
const value = object.data.join();

console.log(value);

Push each value into an array and then use .join() to make a comma-delimited string.

const response = {
  chartstatus: [{
      abs_status: "Bekerja",
      total: "12"
    },
    {
      abs_status: "Tanpa Keterangan",
      total: "5"
    },
    {
      abs_status: "Hari Libur",
      total: "1"
    }
  ]
}

let nama_status = [];
let jumlah = [];

$.each(response.chartstatus, function(key, value) {
  status = value.abs_status.toString();
  nama_status.push('"' + status + '"');
  jumlah.push(value.total)

});
console.log(nama_status.join(', ')); 
console.log(jumlah.join(', '));

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

As simple as this - make an array from this objects

var param = '{"key1":"value1","key2":"value2"},{"key3":"value3"}'; 
var obj = JSON.parse('[' + param + ']');
var objA = obj[0];
var objB = obj[1];

You can do the following,

data = [
  {'id': 1,
  'name': 'book',
  'colors': 'red, yellow, blue'
  },
  {'id': 2,
  'name': 'book',
  'colors': 'red, yellow, blue'
  }
 ];
 
 ret = data.map((item) => {
   return {...item, colors: item.colors.split(',').map(item => item.trim())};
 })

console.log(ret);

var json = [];
var to = '[email protected],[email protected],[email protected]';
var toSplit = to.split(",");
for (var i = 0; i < toSplit.length; i++) {
    json.push({"email":toSplit[i]});
}

Just adding all the steps:-

1). Taking your JSON in a variable.

data = {"url":"http://example.com/main.aspx","report_template_id":"1","interval_secs":"86400","analysis_type":"lite","email":"[email protected]","alerts":["num_domains","med_load_time","avg_load_time","tot_req"]};

2). Parse JSON data to object. Assuming the JSON is a string initially do a typeof(data) to be clear.

data = JSON.parse(data);

3) Change list of alerts to comma separated values

data.alerts = data.alerts.join(',');

4) Convert back to string

data = JSON.stringify(data)

So data will look like

{
    "url": "http://example.com/main.aspx",
    "report_template_id": "1",
    "interval_secs": "86400",
    "analysis_type": "lite",
    "email": "[email protected]",
    "alerts": "num_domains,med_load_time,avg_load_time,tot_req"
}

Note:- If you will just say join() then also it will work, because default delimiter is , only, just to clarify I have given that.

Use Object.keys with Array#map

The Object.keys() method returns an array of a given object's own enumerable properties.

The map() method creates a new array with the results of calling a provided function on every element in this array.

var input = {
  "data": {
    "source1": "source1val",
    "source2": "source2val"
  }
};
var output = Object.keys(input.data).map(function(k) {
  return input.data[k];
}).join(',');
console.log(output); //manipulated object
console.log(input); //Original object

<script src="http://gh-canon.github.io/stack-snippet-console/console.min.js"></script>

This is an invalid notation - either JavaScript object or JSON. If you can fix your input or can make someone fix it, then it is definitely better to make your data source be valid.

However, sometimes we have to work with wrong data (external providers etc.), then you can make it a valid JSON array by adding a couple brackets in the beginning and the end:

var str = '{ "name":"a",  "surname":"b" }, {  "name":"c",  "surname":"d" }, {  "name":"e",  "surname":"f" }';
var arr = JSON.parse("[" + str + "]");
//console.log(arr);

for (var i = 0; i < arr.length; i++)
{
  console.log("Name #" + (i + 1) + ": " + arr[i].name);
  console.log("Surname #" + (i + 1) + ": " + arr[i].surname);
}

It can look a little bit hacky, but it is the best thing you can do when you have to work with such input.
It looks much better than trying to split an object by commas manually, at least for me.

var jsonfied = {
    names: names.replace( /,$/, "" ).split(",").map(function(name) {
        return {name: name};
    })
};

result of stringfying jsonfied:

JSON.stringify( jsonfied );

{
    "names": [{
        "name": "Mark"
    }, {
        "name": "Matthew"
    }, {
        "name": "Luke"
    }, {
        "name": "John"
    }]
}

Live DEMO