Today I Learned

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553 posts by joshbranchaud @jbrancha

Get Random Images From Unsplash

Here is an image from

random images

I don’t know what image I am showing you though. That’s because the URL being sourced for the above image is This tells unsplash to randomly serve us a 1200x600 image. Try refreshing the page and you’ll see that it is different each time. Cool!

This is a bit of a novelty, but could be useful on occasion. The Gatsby docs use it to great effect in a tutorial so as to not get caught up with the details of serving a specific image.

Easy Date Comparison With DayJS

Let’s say my application fetches dates from the server which come back in string form as "YYYY-MM-DD" and I’d like to know if those dates already passed. This can be done easily by wrapping dates in DayJS and using its comparison functions.

import dayjs from 'dayjs';

const today = dayjs(new Date());
const pastDate = dayjs("2018-10-22");
const futureDate = dayjs("2022-01-01");

// => true
// => false

The dayjs() function can be used to construct DayJS date objects from Date objects and strings. These can then be compared with functions like isBefore() and isAfter().

Make A Block Of Text Respect A New Line

Generally when we fill a div tag full of text, it will display it one long strand irrespective of any line breaks that are included. This is a great default, but not necessarily what we want when we are displaying text from another source, such as our users.

We can convince a block of text to respect new lines by adding a couple CSS properties.

.multiline-text {
  word-wrap: break-word;
  white-space: pre-line;

The first rule, word-wrap: break-word, ensures that long lines of text uninterrupted by new lines respect the boundaries of our wrapping element. The second rule, white-space: pre-line, handles squashing of extra white space and respecting of new lines.

See a working example here.

Formik's Validation Schema As A Function

The most straightforward way to use Formik’s validationSchema is to provide it with a Yup object defining your form’s validations.

const MyComponent = withFormik({
  // ...

  validationSchema: yup.object().shape({
    email: yup.string().required(),
    feedback: yup.string().required(),
  // ...

There may be a point at which you need access to the props being passed to MyComponent in order to construct the proper set of validations. Formik supports this by allowing validationSchema to be a function.

const MyComponent = withFormik({
  // ...

  validationSchema: (props) => {
    let emailSchema;
    if(props.allowAnonymous) {
      emailSchema = yup.string();
    } else {
      emailSchema = yup.string().required();

    return yup.object().shape({
      email: emailSchema,
      feedback: yup.string().required(),
  // ...

When validationSchema is a function, its first argument is the set of props passed to that component.

Convert A String To A Timestamp In PostgreSQL

If you have a string that represents a point in time, there are a couple ways that you can convert it to a PostgreSQL timestamptz value.

If the string is in ISO 8601 format, then it can be simply cast to timestamptz.

> select '2018-10-24'::timestamptz;
 2018-10-24 00:00:00-05

A more general purpose approach is to use the to_timestamp function.

> select to_timestamp('2018-10-24', 'YYYY-MM-DD');
 2018-10-24 00:00:00-05

The first argument is our string-to-be-converted in whatever format. The second argument is another string describing in what format that string is.

Note: Both of these approaches produce a timestamptz value.

Compile ReasonML With An OCaml Package Using Dune

In Compile Reason To Native With Dune, I showed how to compile a basic ReasonML file as a native executable using Dune.

Any non-trivial program will likely involve pulling in an OCaml dependency. For example, you may want to pull in Lwt. Assuming this package is available, whether you’ve manually downloaded it via opam or used something like esy, you’ll want to let Dune know that Lwt is an available library.

;; dune
 (name hello_reason)
 (libraries lwt lwt.unix))

The modules in the Lwt package will now be globally available to your Reason code.

let () = {
    Lwt_io.printf("Hello, Reason!\n")

When Dune builds your code, it will include and compile Lwt.

See a full example here.

Show What Is In A Git Stash

Usually when I want to inspect anything in git, I’ll use git show with a specific ref. This can even be done with stash refs.

$ git stash list
stash@{0}: WIP on ...
stash@{1}: Some commit on ...

$ git show stash@{0}
# ...

The git-stash command has a built-in way of showing stashes that will save you from having to type out the somewhat awkward stash@{n} ref.

$ git stash show 1

This will show you the stash@{1} ref. You can also omit a number which will show you the latest stash (stash@{0}).

See man git-stash for more details.

Jump Back To The Latest Jump Position In Vim

Any sort of jump motion, such as gg or 121G, will be recorded in your Vim session’s jump list. If you’d like to return to the latest position in your jump list, you can tap '' within normal mode.

You can then tap '' again and you’ll be returned to the position you were just at. This is great if you want to toggle back and forth between your current position and the previous position.

If the latest jump position is not within the current buffer, then you will be jumped to the initial cursor position at the top of the file.

You can learn more about this by reference :help jump-motions, :help jumplist, and :help ''.

h/t Jake Worth

Compile ReasonML To Native With Dune

Dune is “a composable build system for OCaml” with out-of-the-box support for ReasonML. Dune can be used for a lot of things, but in simplest terms it can be used to compile ReasonML programs into native executables.

Considering the following ReasonML program.

/* */
print_endline("Hello, Reason!")

We can then create the following Dune build file.

;; dune
 (name hello_reason))

If we then run dune build hello_reason.exe, then Dune will compile the into a hello_reason.exe executable that can be found in build/default. Run it and see the output.

Read more in the Quickstart guide.

Doing Date Math In MySQL

MySQL has an array of functions for interacting with date and datetime values. If you’d like to do math with a date to compute a date in the future or the past, you can use the DATE_ADD() and DATE_SUB() functions.

mysql> select now() Now, date_add(now(), interval 10 minute) '10 Minutes Later';
| Now                 | 10 Minutes Later    |
| 2018-10-18 15:53:29 | 2018-10-18 16:03:29 |

mysql> select now() Now, date_sub(now(), interval 9 day) '9 Days Earlier';
| Now                 | 9 Days Earlier      |
| 2018-10-18 15:54:01 | 2018-10-09 15:54:01 |

There are equivalent ADDDATE() and SUBDATE() functions if you prefer.

Check out the docs for more details.

Get The Time Zone Of The Client Computer

The resolvedOptions function on Intl.DateTimeFormat.prototype provides a number of pieces of information about the client computer. It includes information such as the locale and the numbering system. It also has the time zone for that machine.

Try running this line of JavaScript in your own browser.

$ Intl.DateTimeFormat().resolvedOptions().timeZone

When I run it, I get America/Chicago.

You can use this within your client-side code as a way of determining in which time zone your users are.

Git: Stash Everything

Running the git stash command will take all the changes to tracked files in the working directory and stash them away. You can tack on the --include-untracked flag to make sure that untracked files are also included in the stash.

You can take this a step further with the --all flag. This will stash everything included files that you’ve told git to ignore.

You probably don’t want to do this. I ran this command and realized after the command hung for about 10 seconds that I had just stashed the node_modules directory.

See man git-stash for more details.

CSS !important Is Not Supported By Inline Styles

You can get pretty far with React’s inline styling of components. There are however a few limitations. One such limitation is that the !important clause is not supported.

If you try applying !important to an inline style like so:

<div style={{ color: "red !important" }}>
  My div

You’ll be disappointed when you open up the browser and inspect that div tag. The color rule will be ignored and excluded from the output html.


Show Git Changes For Files That Match A Pattern

The git show command allows you to view the changes associated with a reference, such as a commit sha (e.g. git show 86748aacf14e).

Consider a commit that has changed a bunch of JS files as well as two CSS files. If we run git show abcd1234, we will see all of the changes to each file which can result in quite a bit of noise. What if we only want to view the changes to the CSS files?

We can instruct the command to only show changes to files that match a pattern by tacking that pattern on to the end.

$ git show abcd1234 *.css

Alternatively, we could scope the output of the command to the files that live in a certain directory.

$ git show abcd1235 src/css

Wrap A ReasonReact Component For Use In JavaScript

Consider the following ReasonReact component for displaying a greeting.

let component = ReasonReact.statelessComponent("Hello");

let make = (~name, _children) => {
  render: _self =>
    <p> {ReasonReact.string("Hello, ")} {ReasonReact.string(name)} </p>,

If we will just be using this component in a ReasonML context, then that is all we need.

If we want this component available for use in a JavaScript file, we have a little more work to do. We need to define the shape of the component’s props using a bucklescript directive and then wrap the component so that it gets appropriate exported for a JavaScript context.

Here is what that looks like.

[@bs.deriving abstract]
type jsProps = {name: string};

let default =
  ReasonReact.wrapReasonForJs(~component, jsProps =>
    make(~name=jsProps->nameGet, [||])

Our only prop is name which is a string. The wrapReasonForJs function and accompanying binding to default mean that we can import the compiled JS-equivalent like so:

import Hello from 'src/components/';

See the docs for more details on Reason/JS interop.

Format The Current ReasonML File Within Vim

I’m editing a .re file within Vim. I haven’t yet wired up refmt to something like ALE for automatic formatting on save. By the time I’m done with my changes, indentation is a mess.

I can still take advantage of refmt to clean up my file.

:!refmt --in-place %

Running that command in Vim will cause the current file to be formatted.

How does it work?

It shells-out to refmt which does all the heavy lifting. The --in-place flag means that the target file will be re-written by the formatted result. The % is a handy Vim shorthand for the path and name of the current file.

Animate Smoothly Between Two Background Colors

CSS animations allow you to set up simple rules that the rendering engine can then apply to create smooth transitions between style states.

To smoothly transition between two background colors, we can create a keyframes at-rule with a fitting name (e.g. pulse).

@keyframes pulse {
  0% {
    background-color: red;
  50% {
    background-color: blue;
  100% {
    background-color: red;

Over the course of a single animation, this set of rules will start the background color at red, transition to blue 50% of the way through, and then back to red again.

We can then apply this animation within any of our CSS class definitions.

.square1 {
  animation: pulse 2s infinite;
  width: 100px;
  height: 100px;

Anything with a class of square1 will have a width and height of 100px as well as a pulsing background color.

Find The Process Using A Specific Port On Mac

The netstat utility is often recommended for finding the PID (process ID) bound to a specific port. Unfortunately, Mac’s version of netstat does not support the -p (process) flag. Instead, you’ll want to use the lsof utility.

$ sudo lsof -i tcp:4567

Running this will produce a nicely formatted response that tells you several pieces of information about the process bound to :4567 including the PID.


Enable Breadcrumbs For VS Code 1.26 Release

The latest release of Code (Version 1.26) brings about a lot of exciting new features — including file breadcrumbs at the top of each file editor view.

Breadcrumbs feature in action

By default this feature is not enabled. You can enable it from User Settings (Cmd+Shift+P, ‘User Settings’). From the User Settings view, you can search for breadcrumbs and you’ll see the following item:

  "breadcrumbs.enabled": false,

Use the pencil to override it to true and you’ll have that trail of breadcrumbs waiting for you.

Grep For Files With Multiple Matches

The grep utility is a great way to find files that contain a certain pattern:

$ grep -r ".class-name" src/css/

This will recursively look through all the files in your css directory to find matches of .class-name.

Often times these kinds of searches can turn up too many results and you’ll want to pare it back by providing some additional context.

For instance, we may only want results where @media only screen also appears, but on a different line. To do this, we need to chain a series of greps together.

$ grep -rl "@media only screen" src/css |
    xargs grep -l ".class-name"

This will produce a list of filenames (hence the -l flag) that contain both a line with @media only screen and a line with .class-name.

If you need to, chain more grep commands on to narrow things down even farther.

See man grep for more details.

Print The Relative Path Of The Current File

As a project grows and there are many directories and long path names, Vim will not always have enough room to display the full relative path of the current file. It may have to cut off the initial directories which can make it hard to know where you are.

You can hit Ctrl-g to quickly display the relative path of the current file below the status bar.

example of using ctrl-g

See :help Ctrl-g for more details.

Aborting Git Commits And Rebases Within Vim

When you are amending a commit or doing an interactive rebase of a series of commits, Vim will be open to a buffer full of content related to the respective action. Normally, you’ll make some changes, save the buffer, and then quit — Git will take over from there by processing the commit or rebase.

What if you find yourself in this situation and you want to cancel the commit or rebase? Simply quitting with text already in the buffer will be interpreted by Git as a signal to go ahead and process the commit/rebase.

So, how do we quit without confirming the action?

Vim allows you to quit with an error code.


This means that irrespective of the content of the buffer, Vim will signal to Git with an error code to not process the commit or rebase, effectively aborting the action.

See :help cq for more details.

Is This A Directory Or A File -- ReasonML

When compiling ReasonML natively, we have access to a variety of additional modules including the Unix module. We can interact with directories and files using functions on Unix.

let current_dir = Unix.opendir(Unix.getcwd());
let first_file = Unix.readdir(current_dir);
/* is first_file a directory or a file? */

Here we open the current working directory, grab the first thing out of that directory — maybe it’s a file, maybe it’s a directory, maybe it is something else. Lastly, we close the directory.

let current_dir = Unix.opendir(Unix.getcwd());
let first_file = Unix.readdir(current_dir);

switch(Unix.stat(first_file)) {
| Unix.stats({ st_kind: Unix.S_REG }) => print_endline("Regular File")
| Unix.stats({ st_kind: Unix.S_DIR }) => print_endline("Directory")
| Unix.stats({ st_kind: Unix.S_LINK }) => print_endline("Link")
| Unix.stats({ st_kind: Unix.S_SOCK }) => print_endline("Socket")
| _ => print_endline("Something else")


There are a variety of kinds of files to switch on. Here, we are switching on Regular Files, Directories, Links, and Sockets. Everything else falls through.

See the Unix module docs for more details.

For When That Escape Key Is Hard To Reach

Using Vim to its full potential requires using just about every key on the keyboard. Of all those keys, the escape key is particularly important. It’s how you get back to normal node.

If you find the escape key hard to reach or your laptop vendor decided you don’t need it anymore, what’s the alternative?

There is a built-in alternative:


Note: If your <Esc> key is hard to hit on your keyboard, train yourself to use CTRL-[.

See :help Ctrl-[ for more details.


Stream A File Line By Line In ReasonML

We can use the Stream module in ReasonML to read a file getting each line on demand. Doing this requires two key insights. First, we can open a file as an input channel. Second, we can turn an input channel into a stream using Stream.from.

let file_in_channel = Pervasives.open_in('file.txt');

let file_stream =
  Stream.from(_i => {
    switch(Pervasives.input_line(file_in_channel)) {
    | line => Some(line)
    | exception(End_of_file) => None

file_stream |> Stream.iter(line => do_something(line));

The Pervasives module (which is open by default and is only prefixed above so as to be explicit) allows us to open the named file as an input channel with open_in. It also allows us to read lines off that file with input_line. We use Stream.from to create a custom stream that consumes the input channel line by line using input_line. We either get some line or we hit the end of the file. Lastly, we can do whatever we want with the stream, such as iterate over it.

See the docs for Pervasives and Stream for more details.

Add The VSCode CLI To Your Path

Visual Studio Code has a command line tool that can do a bunch of things. Perhaps the most common is opening up the current directory from the command line.

First, you need to add code to your path. This can be done from within Code itself.

Hit Cmd+Shift+p to pop open the command palette. Then start typing Shell Command ... until the Shell Command: Install "code" command in shell PATH option appears. Select this and Code will add code to your path.

Try code . to open the current directory or run code --help for more details on what’s available.

Making Things Mutable In ReasonML

In ReasonML, things that we create with let are immutable — which means that we can’t change them.

let num = 5;

Once num is bound to 5 it is stuck with that value for the duration of it’s scope.

ReasonML doesn’t completely restrict us to immutability though. The ref construct allows us to bind a variable to a sort of box that holds a value. We can then look in the box and change what is in the box.

let num = ref(5); /* put 5 in the box */

Js.log(num^); /* use `^` to look in the box */

num := 3; /* remove 5, put 3 in the box */

We use ref to bind our variable to a box with some initial value. The := assignment operator allows us to change what’s in the box. Anytime we want to refer to what’s in the box, we postfix our variable with ^.

Also of note: while list instances are not mutable, array instances are.


Generate Starter ReasonML Projects With bsb

With the latest bs-platform installed, you should have access to the bsb command which contains a number of options — including -themes.

$ bsb -themes
Available themes:

This is a list of themes (read: templates) that can be used to generate a starter project.

For example, if you’d like a basic project structure geared toward writing Reason, run the following:

$ bsb -init my-project -theme basic-reason

Or if you’d like to get started with reason-react, give this a try:

$ bsb -init my-reason-react-project -theme react


List Stats For A File

The ls command is good for listing files. Tacking on the -la flags gives you a bunch of info about each of the listed files. To get even more info, we can use the stat command.

$ stat
16777220 143994676 -rw-r--r-- 1 jbranchaud staff 0 53557 "Jul 14 14:53:44 2018" "Jul 10 14:54:39 2018" "Jul 10 14:54:39 2018" "Jul 10 14:54:39 2018" 4096 112 0

That’s definitely more info, but it is unlabeled and a lot to parse. We can improve the output with the -x flag.

$ stat -x
  File: ""
  Size: 53557        FileType: Regular File
  Mode: (0644/-rw-r--r--)         Uid: (  501/jbranchaud)  Gid: (   20/   staff)
Device: 1,4   Inode: 143994676    Links: 1
Access: Sat Jul 14 14:53:44 2018
Modify: Tue Jul 10 14:54:39 2018
Change: Tue Jul 10 14:54:39 2018

See man stat for more details.


Dropping Commits With Git Rebase

I’ve been warned enough times about the potential dangers of git reset --hard ... that I always second guess myself as I type it out. Is it git reset --hard HEAD or git reset --hard HEAD~?

If the working directory and index are clean, then there is another way to remove commits. A way that gives me more confidence about what exactly is being removed.

Doing an interactive rebase gives you a number of options. One of those options is d (which stands for drop).

$ git rebase -i master

This pulls up an interactive rebase with all commits going back to what is on master — great for when working from a feature branch.

pick 71ed173 Add Create A Stream From An Array as a reasonml til
pick 80ac8d3 Add some clarity by distinguishing var names
d 4f06c32 Add Data Structures With Self-Referential Types as a reasonml til
d 01a0e75 Fix the name of this file

Adding d next to the commits you want to get rid of and saving will drop those commits. The great part is that there is zero ambiguity about which ones are being dropped.

h/t Jake Worth

Data Structures With Self-Referential Types

ReasonML has a powerful type system that allows us to create types that represent all sorts of things. For data structures like linked lists, we need a sort of recursive type, a type that can reference itself — a self-referential type.

Reason’s type system allows us to define types that reference themselves. Combine that with type arguments and variants — we can create a type definition to represents something like a linked list.

type linked_list('a) =
  | Empty
  | Node('a, linked_list('a));

A linked list is a chain of nodes. It can be an empty list, hence the first part of the variant. Otherwise, it is a node that has some data and then points to another linked list (chain of nodes).

The 'a part is a type argument. When creating a linked list, we can decide what type the 'a will be. Here is an int-based linked list:

let my_list: linked_list(int) = Node(25, Node(27, Empty));
/* my_list = [25] => [27] => [] */

Create A Stream From An Array In ReasonML

There are functions in the Stream module for turning a variety of data structures into streams — lists, input channels, etc.

What if you have an array?

The Stream.from function lets you define a function for custom fitting data structures into streams. Let’s take a look:

let pokemons = [| "bulbasaur", "charmander", "squirtle" |];

let poke_stream: Stream.t(string) =
  Stream.from(i =>
    switch (pokemons[i]) {
    | pokemon => Some(pokemon)
    | exception (Invalid_argument("index out of bounds")) => None

The function takes the current index and needs to either return Some('a) with the corresponding value or None if the stream is empty.

With that, we now have a stream on which we can invoke any of the stream functions.

switch ( {
| pokemon => print_endline(Printf.sprintf("Next Pokemon: %s", pokemon))
| exception Stream.Failure => print_endline("No pokemon left")

Dynamically Create A Printf String Format

Formatting a string with Printf requires defining a format for that string.

let str = Printf.sprintf("%6s", "dope");
/* str => "  dope" */

The format is the first argument. At compile-time it is interpreted as a format6 type value.

So, what if you want a dynamically created format value? Simply concatenating some strings together won’t do it because then the type will be string and that’s not going to compile.

The Scanf.format_from_string function can help.

let some_num = 6;
let format_str = "%" ++ string_of_int(some_num) ++ "s";
let format = Scanf.format_from_string(format_str, "%s");

let str = Printf.sprintf(format, "dope");
/* str => "  dope" */

We can convert our string that has the appearance of a format into an actual format6 type. To do this, we have to tell format_from_string what types each of the formats is going to have — hence the second argument %s.


Break Out Of A While Loop In ReasonML

The while construct is a great way to loop indefinitely. You may eventually want to break out of the loop. For that, you are going to need to invalidate the while condition. One way of going about this is creating a mutable ref, changing it from true to false when a certain condition is met.

let break = ref(true);

while (break^) {
    switch (Unix.readdir(currentDir)) {
    | exception End_of_file => break := false
    | item => print_endline(item);

Here we have a mutable ref called break which starts as true. This is our while condition. Its actual value can be referenced by appending the ^ character — hence break^. Once a certain condition is met inside our while block, we can set break to false using the := operator.

The above code snippet can be seen in full details here.


Disable Swipe Navigation For A Specific App On Mac

Mac’s touch pad has a bunch of handy swipe gestures, including swiping two fingers to the left or the right to navigate backward or forward. This particular gesture can be globally enabled and disabled. I find it useful for most apps and a pain in a few apps, such as Google Chrome.

From the terminal we can disable it for a specific app (like Google Chrome):

$ defaults write AppleEnableSwipeNavigateWithScrolls -bool FALSE

Restart the target application, in my case Chrome. The left and right swipe navigation will no longer be triggered.


Create A Cancelable Promise With PCancelable

If an async task takes a really long time and we find out in the middle of its execution that we no longer want the results of that activity, it would be nice to be able to cancel that promise. There was a proposal for cancelable promises, but it has since been withdrawn. There is an alternative though.

The p-cancelable package provides a promise wrapper that acts as a cancelable promise.

import PCancelable from 'p-cancelable';

const fetchPromise = new PCancelable((resolve, reject, onCancel) => {
  setTimeout(() => {
    resolve({ ok: true, data: [1, 2, 3] });
  }, 10000);

  onCancel(() => {
    console.log('Promise is being canceled');

fetchPromise.then(response => {
  console.log('Promise Resolved: ',;
}).catch(err => {
  console.log('Promise Rejected: ', err);


We can create a promise in a familiar manner. We get an additional argument — the onCancel function. This is a function that will be executed if cancel is called before the promises has resolved or rejected. In the above example, the fetchPromise.cancel() line will be invoked before the setTimeout resolves. The promise will be canceled, causing a rejection which will push us into the catch handler.

Forcing A Child Remount With The Key Prop

There are a couple prop names that have reserved usage in React. One of those is key. We generally use key when we are rendering a list of things. It is a way of uniquely identifying each element in a list so that React minimizes re-rendering when parts of the list change.

We can flip this on its head and utilize key as a way of forcing a remount and re-render of a child component.

Imagine I have a component that does a number of things including rendering a component with some internal state, such as a counter.

class MyComponent extends React.Component {
  state = {
    remountKey: (new Date()).getTime(),

  resetCounter = () => {
      remountKey: (new Date()).getTime(),

  render() {
    return (
        {/* some other stuff in my component */}

        <Counter key={this.state.remountKey} />
        <button onClick={this.resetCounter}>Reset Counter</button>

I can force this Counter component to remount, thus resetting its state by passing it a new key value. The button can trigger this by updating the remountKey value.

Create An Object With No Properties

When you call new Object or even just instantiate an object with {}, you are creating an object that uses the Object prototype. This means it inherits from Object.prototype.

You can deliberately create an object with no properties by making sure that it does not inherit Object.prototype.

> const propertylessObject = Object.create(null);

> propertylessObject.__proto__

Unlike most objects that we encounter as we write JavaScript, this object we created with Object.create(null) has no properties including no __proto__.

See Object.create and Object.prototype for more details.

Allow HTTPS Through Your UFW Firewall

UFW — Uncomplicated Firewall — is just what is sounds like. I have it running on a DigitalOcean box and it is only letting through traffic on ports 80 (HTTP) and 22 (SSH). I am setting up SSL for a domain hosted on this box which means I need to also let through traffic on 443 (HTTPS).

The allowed ports can be checked with the status command:

$ sudo ufw status

Status: active

To                         Action      From
--                         ------      ----
OpenSSH                    ALLOW       Anywhere
Nginx HTTP                 ALLOW       Anywhere
OpenSSH (v6)               ALLOW       Anywhere (v6)
Nginx HTTP (v6)            ALLOW       Anywhere (v6)

As we can see, HTTPS has not yet been allowed by ufw. We can allow HTTPS traffic with the allow command.

$ sudo ufw allow https

Check the status again and see that HTTPS is now included in the list.


h/t Dillon Hafer

Turn Off Console Error Messages In A Test

I’m using Jest to test a React component that requires a prop via PropTypes. In one of my tests, I want to test that component when the required prop is excluded. The side effect of doing this is that my test output gets cluttered with the PropType warning.

The thing to do is silence the error message during that test.

it('renders a component without a required prop', () => {
  const originalError = console.error;
  console.error = jest.fn();

  // test code here
  expect(shallow(<My Component />)).toDoSomething;

  console.error = originalError;

We can silence console.error by temporarily replacing it with a Jest-mocked function and then putting it back at the end of the test.

Spelunking Through Components With Enzyme's Dive

Most of the components we write have other components nested in them.

const Hello = ({ name }) => <h1>Hello {name}!</h1>;

const HelloContainer = (props) => (
    <Hello {...props} />

If we are to shallow render the above component using Enzyme, we’ll only see things one layer deep:

const wrapper = shallow(<HelloContainer name="World" />);
// wrapper ~= <div><Hello name="World" /></div>

If we’d like to explore a particular child of the rendered component further, we can do a little find and dive.

const wrapper = shallow(<HelloContainer name="World" />);
const helloWrapper = wrapper.find(Hello).dive();
expect(helloWrapper.text()).toEqual("Hello World!");

This allows us to make pinpoint assertions about how our components render without mounting the entire thing.

See a live example here.

h/t Vidal Ekechukwu

Use React 16 With Gatsby

Gatsby, the blazing fast static site generator for React, is tied to React 15.6. If you’ve been using React 16+ for a while, then this may come as a bit of a buzzkill.

Fortunately, there is a Gatsby plugin that lets you use React 16 with a Gatsby v1 site — gatsby-plugin-react-next.

Add it the plugin as a dependency:

$ yarn add gatsby-plugin-react-next

Then add it to the list of plugins in gatsby-config.js:

plugins: [`gatsby-plugin-react-next`];


Add Fab Icons To Your Site With FontAwesome 5

Check out the latest version of FontAwesome.

Version 5 has been re-written and re-designed completely from scratch.

One part of the rewrite is that brand icons have been organized into their own collection. Whether you are using the full suite or just the brands bundle, you’ll be referencing these icons with the fab class name.

<i class="fab fa-react" />

This will give you a React icon.

Check out all the different brand icons here.

Avoid The Notch With SafeAreaView On React-Native

iOS devices, especially the iPhone X, have areas of the screen that are cut off in ways that present quite a challenge to developers. One of the easiest ways to deal with rounded corners and the notch when developing for iOS is to avoid them all together.

The purpose of SafeAreaView is to render content within the safe area boundaries of a device.

import { SafeAreaView, View, Text } from 'react-native';

const App = () => {
  return (
    <SafeAreaView style={{ flex: 1, backgroundColor: "#e6e6e6" }}>
        <Text>Safely rendered in the visible area of the device!</Text>

The unsafe area can be styled however you like and everything inside <SafeAreaView> will be pushed into the visible area of the screen.

h/t Dillon Hafer


Dynamically Create HTML Elements

An HTML element can be created with a string that matches a recognized entity.

const Paragraph = 'p';
return <Paragraph>Some paragraph content</Paragraph>

This means we can dynamically create HTML elements such as headers:

const H = ({ level, ...props }) => {
  const Heading = `h${Math.min(level, 6)}`;
  return <Heading {...props} />;

return (
    <H level={1}>Header 1</H>
    <H level={2}>Header 2</H>
    <H level={5}>Header 5</H>

With some inspiration, I’ve created a live example here.

create-react-app Has A Default Test Setup File

In Configure Jest To Run A Test Setup File, I pointed to a way of configuring Jest in either the package.json or jest.config.js file with the setupTestFrameworkScriptFile value.

In a create-react-app project, this is not an option because setupTestFrameworkScriptFile is not one of the permitted config values for Jest.

There is a built-in value which happens to match what was recommended in the above post — <rootDir>src/setupTests.js.

This means that there is no configuration required. Instead, just create a setupTests.js file in the src directory of your CRA project and add any framework setup you need there. That file is already configured to run when you invoke yarn test.


Configure Jest To Run A Test Setup File

Jest can be configured to run a setup file before each test. This is useful for configuring your testing framework in a single place, rather than in each test file.

This setup file can be specified in package.json (or jest.config.js).

// package.json
  // ...
  "jest": {
    "setupTestFrameworkScriptFile": "<rootDir>src/setupTests.js"

The setupTestFrameworkScriptFile points to a test setup file at the specified location rooted at <rootDir> (the root of your project).

This kind of setup is helpful for something like Enzyme that needs to be configured with a specific adapter for use throughout your tests.