Adding an object to an array of objects with splice

If you just want to add a value to the end of an array then the push(newObj) function is easiest, although splice(...) will also work (just a bit trickier).

var event_id = [{"0":"e1"}, {"0":"e2"}, {"0":"e4"}];
event_id.push({"0":"e5"});
//event_id.splice(event_id.length, 0, {"0":"e5"}); // Same as above.
//event_id[event_id.length] = {"0":"e5"}; // Also the same.
event_id; // => [{"0":"e1"}, {"0":"e2"}, {"0":"e4"}, {"0":"e5"}]; 

See the excellent MDN documentation for the Array object for a good reference of the methods and properties available on arrays.

[Edit] To insert something into the middle of the array then you'll definitely want to use the splice(index, numToDelete, el1, el2, ..., eln) method which handles both deleting and inserting arbitrary elements at any position:

var a  = ['a', 'b', 'e'];
a.splice( 2,   // At index 2 (where the 'e' is),
          0,   // delete zero elements,
         'c',  // and insert the element 'c',
         'd'); // and the element 'd'.
a; // => ['a', 'b', 'c', 'd', 'e']

ES6 solution with spread operator:

event_id=[{"0":"e1"},{"0","e2"},{"0","e4"}];
event_id = [...event_id,{"0":"e5"}]

or if you do not want to mutate event_id

newEventId = [...event_id,{"0":"e5"}]

UPDATE: to insert object after a specific index or object key or object value respectively you can:

const arr = [{a:1},{b:2},{c:3},{d:4}]

arr.reduce((list,obj,index)=>index===1 ? [...list,obj,{g:10}] : [...list,obj], [])
arr.reduce((list,obj)=>Object.keys(obj)[0]==='b' ? [...list,obj,{g:10}] : [...list,obj], [])
arr.reduce((list,obj)=>Object.values(obj)[0]===2 ? [...list,obj,{g:10}] : [...list,obj], [])

// output:  [ { a: 1 }, { b: 2 }, { g: 10 }, { c: 3 }, { d: 4 } ]

Since I want to add the object in the middle of the array, I ended with this solution:

var add_object = {"0": "e5"};
event_id.splice(n, 0, add_object); // n is declared and is the index where to add the object