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# v2 Upgrade Guide
## Common changes
This page covers a few of the most common problems people face when updating from v1 to v2. For a more detailed list of changes, look at the [change log for version 2.0.0](https://date-fns.org/docs/Change-Log).
### Camel case naming schema
Function submodules now use camelCase naming schema:
```javascript
// Before v2.0.0
import differenceInCalendarISOYears from 'date-fns/difference_in_calendar_iso_years'
// v2.0.0 onward
import differenceInCalendarISOYears from 'date-fns/differenceInCalendarISOYears'
```
### New formatting tokens
Starting with v2 `format` and `parse` uses [Unicode tokens].
See [Unicode Tokens doc](https://date-fns.org/docs/Unicode-Tokens) for more details.
### String arguments
Functions now don't accept strings as date arguments. Strings should
be parsed using `parseISO` (ISO 8601) or `parse`.
See [this post](https://blog.date-fns.org/post/we-cut-date-fns-v2-minimal-build-size-down-to-300-bytes-and-now-its-the-smallest-date-library-18f2nvh2z0yal) for more details.
```javascript
// Before v2.0.0
addDays('2016-01-01', 1)
// v2.0.0 onward
addDays(parseISO('2016-01-01'), 1)
```
### Arguments conversion
All functions now implicitly convert arguments by following rules:
| | date | number | string | boolean |
| --------- | ------------ | ------ | ----------- | ------- |
| 0 | new Date(0) | 0 | '0' | false |
| '0' | Invalid Date | 0 | '0' | false |
| 1 | new Date(1) | 1 | '1' | true |
| '1' | Invalid Date | 1 | '1' | true |
| true | Invalid Date | NaN | 'true' | true |
| false | Invalid Date | NaN | 'false' | false |
| null | Invalid Date | NaN | 'null' | false |
| undefined | Invalid Date | NaN | 'undefined' | false |
| NaN | Invalid Date | NaN | 'NaN' | false |
Notes:
- as before, arguments expected to be `Date` are converted to `Date` using _date-fns'_ `toDate` function;
- arguments expected to be numbers are converted to integer numbers using our custom `toInteger` implementation
(see [#765](https://github.com/date-fns/date-fns/pull/765));
- arguments expected to be strings are converted to strings using JavaScript's `String` function;
- arguments expected to be booleans are converted to boolean using JavaScript's `Boolean` function.
`null` and `undefined` passed to optional arguments (i.e. properties of `options` argument)
are ignored as if no argument was passed.
If any argument is invalid (i.e. `NaN` for numbers and `Invalid Date` for dates),
an invalid value will be returned:
- `false` for functions that return booleans (expect `isValid`);
- `Invalid Date` for functions that return dates;
- `NaN` for functions that return numbers;
- and `String('Invalid Date')` for functions that return strings.
See tests and PRs [#460](https://github.com/date-fns/date-fns/pull/460) and
[#765](https://github.com/date-fns/date-fns/pull/765) for exact behavior.
### `null`
`null` now is not a valid date. `isValid(null)` returns `false`;
`toDate(null)` returns an invalid date. Since `toDate` is used internally
by all the functions, operations over `null` will also return an invalid date.
[See #537](https://github.com/date-fns/date-fns/issues/537) for the reasoning.
### `RangeError`
Functions now throw `RangeError` if optional values passed to `options`
are not `undefined` or have expected values.
This change is introduced for consistency with ECMAScript standard library which does the same.
### `TypeError`
All functions now check if the passed number of arguments is less
than the number of required arguments and throw `TypeError` exception if so.
### UMD/CDN
The Bower & UMD/CDN package versions are no longer supported.
### New locale format
See [docs/Locale](https://date-fns.org/docs/Locale).
Locales renamed:
- `en` → `en-US`
- `zh_cn` → `zh-CN`
- `zh_tw` → `zh-TW`
```javascript
// Before v2.0.0
import locale from 'date-fns/locale/zh_cn'
// v2.0.0 onward
import locale from 'date-fns/locale/zh-CN'
```
[unicode tokens]: https://www.unicode.org/reports/tr35/tr35-dates.html#Date_Field_Symbol_Table
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