Is the IF function removing the date object in javascript?-CodePudding

I've spent an hour looking for answers and trying different things so I appreciate any help here.

The following code works great for finding someone's part B effective date. However, when someone's birthday is really on the 1st of a month the 'if' function get's used, and I'm no longer able to format and write the date. It's almost like 'partB_eff' is no longer a date object. (I'm a newbie, so I might just be making this part up.)

I'm getting the error "TypeError: partB_eff.toLocaleDateString is not a function at AutoFill_6_Step_Checklist(Code:24:27)"

How can I resolve this?

let birthday = new Date(e.values[2]);
    //this is a date entered from a google form
let bdayCopy = new Date(birthday);
    //I created this since I'll be using .setMonth(), and I don't want to change the original date of the birhtday
let bday65 = new Date(bdayCopy.setMonth(bdayCopy.getMonth() 780));
    //finds the 65th birthday
let partB_eff = new Date(bdayCopy.setDate(01));
    //find's the Medicare part B effective date (the 1st of the month someone turns 65)

if(birthday.getDate()==1){
    partB_eff = partB_eff.getMonth-1;
    //if the person's birthday is really on the 1st of the month, the part b effective date is the 1st of the month prior. partB_eff must be converted
  }

partB_eff = partB_eff.toLocaleDateString('en-us',{year:"numeric",month: "short",day:"numeric"});
    //format partB_eff so that it looks nice on paper

CodePudding user response：

partB_eff = partB_eff.getMonth-1;

Doesn't do what you think it does. What it does is get the vound function getDate from your date object, and attempt to subtract one from it. In any other language trying to do subtraction on a function would be a type error, but Javascript is Javascript and allows numeric operations on almost any type. A function minus a number in JS is NaN. NaN doesn't have a method called toLocaleString, hence the error.

What's interesting is that you did the same operation correctly above with bdayCopy.setMonth(bdayCopy.getMonth() 780) Just do the same thing here

bdayCopy = new Date(bdayCopy.setMonth(bdayCopy.getMonth()-1));

Also some important concepts. if in Javascript is not a function. if is a keyword that starts a conditional statement. You can't do any of the things you can do with a function with if. You can't call it or assign it to a variable or pass ot as a function argument. Clearly understanding what a function is is something you need to do to be able to work in JS, or frankly any other language.

Finally if you are doing date math in JS I strongly recommend you use a date library like DateFns or Moment. Javascript native date APIs are possibly the worst designed date API of any language ever.