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

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);

Another option is just to use const instead of let, so literal type will be inferred:

const units = 'month';
moment().add(1, units);

Answer 2

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 3

Unfortunately, none of the above answers worked with me! But this only made the charm! :D

const startTime = moment().subtract(this.time.amount as moment.DurationInputArg1, this.time.unit as moment.DurationInputArg2);

Answer 4

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 5

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 6

This answer is building on all the previous answers that specify deprecated reverse syntax of add, subtract, etc functions in moment as the cause of the problem.

I cast the DurationInputArg2 field to unitOfTime.Base since all these methods have this as the ultimate superclass and so it means for add, subtract, startOf, endOf I just need to remember as Base:

 import Base = moment.unitOfTime.Base

 theMoment.subtract(amount, unitString as Base)
 theMoment.startOf(unitString as Base)

The confusing part for most people is that the amount is marked as the error. Fixed when one qualifies the units.