Javascript – Run two commands at the same time in Elm

elmjavascript

In Elm, and specifically with the Elm Architecture when the app first starts the init function can return a Cmd Msg that is executed. We can use this for sending http requests or send a message to native Javascript via Elm ports.

My question is, how can I send multiple commands that should be executed in init?

For example I can do something like:

init : (Model, Cmd Msg)
init =
  (Model "" [], (Ports.messageToJs "Hello JS"))

And I can do something like:

url : String
url =
     "http://some-api-url.com"
...

fetchCmd : Cmd Msg
fetchCmd =
    Task.perform FetchError FetchSuccess fetchTask


init : (Model, Cmd Msg)
init =
  (Model "" [], fetchCmd)

How can I return both commands at same time from init?

I have seen Task.sequence and even Task.parallel but they appear to be good for running multiple tasks, not specifically commands.

Best Answer

Use Platform.Cmd.batch (docs):

init : (Model, Cmd Msg)
init =
  ( Model "" []
  , Cmd.batch [fetchCmd, Ports.messageToJs "Hello JS")]
  )

As of September 2019, this works in 95% of the browsers used globally.

Old compatibility information

ecma-402/1.0 says that timeZone may be undefined if not provided to constructor. However, future draft (3.0) fixed that issue by changing to system default timezone.

In this version of the ECMAScript Internationalization API, the timeZone property will remain undefined if no timeZone property was provided in the options object provided to the Intl.DateTimeFormat constructor. However, applications should not rely on this, as future versions may return a String value identifying the host environment’s current time zone instead.

in ecma-402/3.0 which is still in a draft it changed to

In this version of the ECMAScript 2015 Internationalization API, the timeZone property will be the name of the default time zone if no timeZone property was provided in the options object provided to the Intl.DateTimeFormat constructor. The previous version left the timeZone property undefined in this case.

Javascript – How to get the difference between two arrays in JavaScript

There is a better way using ES7:

Intersection

 let intersection = arr1.filter(x => arr2.includes(x));

For [1,2,3] [2,3] it will yield [2,3]. On the other hand, for [1,2,3] [2,3,5] will return the same thing.

Difference

let difference = arr1.filter(x => !arr2.includes(x));

For [1,2,3] [2,3] it will yield [1]. On the other hand, for [1,2,3] [2,3,5] will return the same thing.

For a symmetric difference, you can do:

let difference = arr1
                 .filter(x => !arr2.includes(x))
                 .concat(arr2.filter(x => !arr1.includes(x)));

This way, you will get an array containing all the elements of arr1 that are not in arr2 and vice-versa

As @Joshaven Potter pointed out on his answer, you can add this to Array.prototype so it can be used like this:

Array.prototype.diff = function(arr2) { return this.filter(x => !arr2.includes(x)); }
[1, 2, 3].diff([2, 3])

Best Answer

Related Solutions

Javascript – Getting the client’s time zone (and offset) in JavaScript

As of September 2019, this works in 95% of the browsers used globally.

Old compatibility information

Javascript – How to get the difference between two arrays in JavaScript

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