未捕获的TypeError：undefined不是函数

Question

I am trying to find the difference between 2 dates in whole months, I found a little script on Stack Exchange, but it keeps giving me an error of "Uncaught TypeError: undefined is not a function".

I get this in chrome, it is a fairly simple HTML page.

It doesn't seem to like the first line of the function (var monthsLeft = to.getMonth() etc...)

(slightly simplified version for clarity:)

<script>
function monthDiff(from, to) {
    var monthsLeft = to.getMonth() - from.getMonth() + (12 * (to.getFullYear() - from.getFullYear()));
    if(to.getDate() < from.getDate()) {
        monthsLeft--;
    }
    return monthsLeft;
  }


function lumpChange() {

        var today = new Date();
        var april5th = new Date();
        var MonthsLeft = 0;

        today = '10/05/2015';
        april5th = '04/05/2015';
        MonthsLeft = monthDiff(today, april5th);  
  }

</script>

Answer 1

Your monthDiff() function looks as though it expects Date objects to be passed as parameters (it uses methods like .getMonth() etc). You are passing strings.

You don't modify Date objects by assigning strings to the variable the object was originally in.

You can define the Date when instantiating it:

function lumpChange(){
    var today = new Date(2015, 10, 05),
        april5th = new Date(2015, 04, 05);

    MonthsLeft = monthDiff(today, april5th); // -6
}

JSFiddle

If you need to set the date/time of a Date object after instantiating it, you can use the setter methods .

Documentation:

• Date object