In JSON, numbers don't need double quotes, so you could just append [ and ] to either end of the string, resulting in the string "[1,4,5,11,58,96]" and you will have a JSON Array of numbers.

You could add brackets to the string, parse the string (JSON.parse) and map (Array#map) the property and the join (Array#join) the result.

var string = '{"name":"Marine Lines","location_id":3},{"name":"Ghatkopar","location_id":2}',
    array = JSON.parse('[' + string + ']'),
    result = array.map(function (a) { return a.location_id; }).join();

console.log(result);

I would use a reduce function as that is used to reduce an array to a single output. If it is the first item (index == 0), then just return the name to the accumulator (which kind of stores the string and appends to it as you go through the array) with quotation marks on either side (escaped with \ but not necessary if using single quotes), else return the accumulator with a comma and a quotation mark after it as well as the name and another quotation mark.

So as you go through the array, the accumulator will be something like this:

•  
• "John"
• "John","Tim"
• "John","Tim","Mike"

and when it gets to the end, it returns it to the variable, so commaSeparatedString == "\"John\",\"Tim\",\"Mike\""

const users = [{
  "name": "John",
  "color": "blue",
}, {
  "name": "Tim",
  "color": "red",
}, {
  "name": "Mike",
  "color": "green",
}];


const commaSeparatedString = users.reduce(function(accumulator, currentValue, index) {
  return index == 0 
    ? '\"' + currentValue.name + '\"'
    : accumulator + ',\"' + currentValue.name + '\"';
}, '');

console.log(commaSeparatedString);

Watch out if you are aiming at integers, like 1,2,3,4,5. If you intend to use the elements of your array as integers and not as strings after splitting the string, consider converting them into such.

var str = "1,2,3,4,5,6";
var temp = new Array();
// This will return an array with strings "1", "2", etc.
temp = str.split(",");

Adding a loop like this,

for (a in temp ) {
    temp[a] = parseInt(temp[a], 10); // Explicitly include base as per Álvaro's comment
}

will return an array containing integers, and not strings.

Strings get into arrays in each object.

let arr = [
  {id: 1,
  name: 'book',
  colors: 'red, yellow, blue'
  },
  {id: 2,
  name: 'book',
  colors: 'red, yellow, blue'
  }
  ]
 
 for (let index in arr) {
   let colorsIntoArr = arr[index].colors.split(',');   
   arr[index].colors = colorsIntoArr;   
 }
 /*
 [
  {
    "id": 1,
    "name": "book",
    "colors": [
      "red",
      " yellow",
      " blue"
    ]
  },
  {
    "id": 2,
    "name": "book",
    "colors": [
      "red",
      " yellow",
      " blue"
    ]
  }
]
*/ 
 
 console.log(arr)

Access object properties using dot notation (e.g. obj.data) and then on the array you can use join to convert to a string with a comma in between.

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

console.log(obj.data.join(', '))

Use the split function which is available for strings and convert the numbers to actual numbers, not strings.

var ar = "'California',51.2154,-95.2135464,'data'".split(",");

for (var i = ar.length; i--;) {
  var tmp = parseFloat(ar[i]);
  ar[i] = (!isNaN(tmp)) ? tmp : ar[i].replace(/['"]/g, "");
}

console.log(ar)

Beware, this will fail if your string contains arrays/objects.