Unable to pass string variable to JS date object

Question

I am trying to pass string variable to JS date object and create date in future. Its easy to do in this way:

var d = new Date();
var year = d.getFullYear();
var month = d.getMonth();
var day = d.getDate();
var my_date = new Date(year + 1, month +1, day +1);

But how I can pass it with string variable, so that I can achieve something like this:

var d = new Date();
var year = d.getFullYear();
var month = d.getMonth();
var day = d.getDate();
var my_variable = 'year + 1, month +1, day +1';
var my_date = new Date(my_variable);

In this case it returns Invalid date

Yeah, it doesn't work that way. Why do you have what is essentially Javascript code as a string as input?! — deceze, Sep 28 '18 at 08:18
As @deceze says, it definitely doesn't work that way. You could use an `eval` statement? — freefaller, Sep 28 '18 at 08:19
string as input is required because I will pass there dynamic data — user3432056, Sep 28 '18 at 08:21
@freefaller Everytime you mention `eval` you should also use: `*WARNING* DO NOT USE EVAL` — Rafael Herscovici, Sep 28 '18 at 08:21
@Dementic - absolutely agree, but can't think of an obvious other solution — freefaller, Sep 28 '18 at 08:23
In order to manipulate dates, I would recommend you to use libraries like https://github.com/date-fns/date-fns https://momentjs.com/ or others — wanjas, Sep 28 '18 at 08:23
Possible duplicate of [Add days to JavaScript Date](https://stackoverflow.com/questions/563406/add-days-to-javascript-date) — Rafael Herscovici, Sep 28 '18 at 08:23
You need a bespoke parser. Not too difficult to write, just a bit tedious. — RobG, Sep 28 '18 at 08:47
why don't you return with an object where you define the addition value? Like `obj = {addToYear: 1, addToMonth: 1, addToDay: 1}`. Then you only have to do eg `new Date (year + obj.addToYear, month + obj.addToMonth, day + obj.addToDay)`. If you want remove days, simply use `addToDay: -5` to remove 5 days. — KarelG, Sep 28 '18 at 08:51

score 2 · Accepted Answer · edited Jun 20 '20 at 09:12

Note

You need to be a little careful with date arithmetic since it's not symmetric and may give odd results, e.g. 29 Feb 2016 + 1 year gives 1 March 2017, and 31 July minus 1 month is 1 July since there is no 31 June, it rolls over to July, and so on.

Given your original requirement, you might consider a parser for your particular format, e.g.

function myParse(s, date) {
  date = date || new Date();
  // Default values object
  var values = {
    year:{sign:1, value:0}, month:{sign:1, value:0}, day:{sign:1, value:0}
  };
  // Tokenise string
  var part = s.toLowerCase().match(/[a-z]+|\d+|[+-]/ig);

  // Process the tokens
  if (part) {
    for (var i=0, iLen=part.length; i<iLen; i++) {
      if (part[i] in values) {
        values[part[i]].sign = part[i+1] == '+'? 1 : -1;
        values[part[i]].value = +part[i+2];
        i += 2;
      }
    }
  }
  // Apply to date
  date.setFullYear(
    date.getFullYear() + values.year.sign  * values.year.value,
    date.getMonth()    + values.month.sign * values.month.value,
    date.getDate()     + values.day.sign   * values.day.value
  );
  
  return date;
}

// Examples
var options = {day:'2-digit',month:'short',year:'numeric'};
console.log('Today is ' + (new Date().toLocaleString('en-gb', options)));

['year+1,month+1,day+1', // one of each
 'year+2,day+2',         // missing parameters
 'day  +  15, month +2', // any order, random whitespace
 'month - 3'             // subtraction

].forEach(function(s) {
  console.log(s + '\n' + myParse(s).toLocaleString('en-gb', options));
});

It's fairly tolerant but you should validate the input. It could be extended to handle time too, and also the end of month issues noted above.

Charlie · Answer 2 · 2018-09-28T08:27:55.597

1

You are inserting Javascript code as a string, which makes no sense to the Date class.

You can have your variables stored in an array to pass them elegantly to Date.

const dates = [year + 1, month +1, day +1]
const my_date = new Date(...dates);

I've read a follow up comment which indicates you'll only be getting strings from wherever you are retrieving the data from. Firstly if the string is in the format of 'year + 1, month + 1, day + 1', it'll never make sense to the Date object. If the string is just 3 numbers separated by a comma, you could split the string at the commas and pass them to the Date object in the same fashion.

const dates = myString.split(',');
const my_date = new Date(...dates);

edited Sep 28 '18 at 08:27

answered Sep 28 '18 at 08:20

Charlie

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3
13

I believe the *problem* is that the values are stored in a single string, not a series/array of useful values – freefaller Sep 28 '18 at 08:24
Yeah, I just noticed that with his follow up comments. If the strings are numbers separated by commas, he could just split the string at commas and parse each element into a number, then pass it to the Date object in the same way. – Charlie Sep 28 '18 at 08:25

score 1 · Answer 3 · answered Sep 28 '18 at 08:21

Unfortunately javascript (and all the languages that I know off hand) do not work like that.

You could use an eval statement instead, which allows you to create the entire instruction as a string and run that string...

var my_variable = 'year + 1, month +1, day +1';
eval('var my_date = new Date(' + my_variable + ');');

HOWEVER using of eval is dangerous, and could open you up to unexpected attacks. Only use it if absolutely necessary, and if you're sure that the string being used is "safe"

Unable to pass string variable to JS date object

3 Answers3