moment().add() only works with literal values

Question

I'm using Moment.js in TypeScript (under Angular 2 if that matters). When I use the add() method with literal values as arguments, it works fine:

moment().add(1, 'month');

However, if I try to replace the units with a string, it fails:

let units:string = 'month';
moment().add(1, units);

with this error:

Argument of type '1' is not assignable to parameter of type 'DurationConstructor'.

What am I doing wrong here?

Answer 1

There are two declarations of add

1. add(amount?: DurationInputArg1, unit?: DurationInputArg2): Moment;
2. Deprecated add(unit: unitOfTime.DurationConstructor, amount: number|string)

When typescript need to match overloads it tries to take closest type. When you've tried to pass units:string, closest match number|string is second overload, because any string will match to this declaration, f.e. 'qwe', or 'rty' of type string, but not DurationArg2 which is expected as second parameter in first declaration.

When you call moment.add(1,'months) it will use first declaration, because it is possible to cast 1 to type of the first argument in first signature, but not in the second.

So to fix this issue and similar, you should say what you exactly want to use.

Example 1. First signature will be used

import * as moment from 'moment'

calculateRangeFromEnd(end: Date, unitsAmount: moment.DurationInputArg1, unitsMeasureMoment: moment.DurationInputArg2): IDateRange {
    return {
      endDate: end,
      startDate: moment(end).subtract(unitsAmount, unitsMeasureMoment).toDate()
    }
  }

Example 2. Second signature will be used

import * as moment from 'moment'

calculateRangeFromEnd(end: Date, unitsAmount: number | string, unitsMeasureMoment: moment.unitOfTimes.DurationConstructor): IDateRange {
    return {
      endDate: end,
      startDate: moment(end).subtract(unitsMeasureMoment, unitsAmount).toDate()
    }
  }

Answer 2

The problem in my case was that I was using mins. The type def for DurationConstructor is

namespace unitOfTime {
type Base = (
  "year" | "years" | "y" |
  "month" | "months" | "M" |
  "week" | "weeks" | "w" |
  "day" | "days" | "d" |
  "hour" | "hours" | "h" |
  "minute" | "minutes" | "m" |
  "second" | "seconds" | "s" |
  "millisecond" | "milliseconds" | "ms"
);

So this works out of the box:

refTime.clone().subtract(15, 'minute')

-k

Answer 3

Another option to the accepted answer is typecasting in the argument. There's really no difference, just figured I'd include this answer as an option. Also unitOfTime can be imported from moment as a module if you want some more brevity.

import { unitOfTime } from 'moment';
import * as moment from 'moment';

option = {val: 30, unit: 'm'}
moment().add( this.querySince.val, <unitOfTime.DurationConstructor>this.querySince.unit )

Answer 4

This problem is caused by deprecated reverse syntax of add, subtract, etc functions in moment library. Below is an excerpt:

moment.d.ts

add(amount?: DurationInputArg1, unit?: DurationInputArg2): Moment;
/**
 * @deprecated reverse syntax
 */
add(unit: unitOfTime.DurationConstructor, amount: number|string): Moment;

In my case I wanted units and value both to be variables so I had to write a function to convert the general string to the acceptable ones ("hours", "minutes", etc.).

function convertToDuration (unit: string): Moment.unitOfTime.DurationConstructor {

if(unit ==  'seconds' || unit ==  'minutes' || unit ==  'hours' || unit ==  'days' || unit ==  'weeks' || unit ==  'months'){
    return unit;
}
else // Default unit is hours
    return 'hours';
}

Now to call this function:

const time = moment().add(expiryTime.value, convertToDuration(expiryTime.unit));

Definition of expiryTime in TypeScript:

interface ExpiryTimeDef {
value: number,
unit: string
}

Answer 5

Deprecated reverse overload add(unit: unitOfTime.DurationConstructor, amount: number|string) creates ambiguity.

You can fix this by defining type of units to be DurationConstructor instead of string:

let units:moment.unitOfTime.DurationConstructor = 'month';
moment().add(1, units);