Javascript | Today I Learned

There's an API in Chrome and Firefox (that I know of) that lets you get localized strings with variable replacements from a JSON file that you provide.

var message = browser.i18n.getMessage("messageContent", target.url);

console.log(message);

Arrays can be copied using spread syntax, introduced in ES6. To illustrate:

let a = [1, 2, 3];

// This won't work.

let b = a;

b[0] = 99;

console.log(a[0], b[0]); // prints "99 99"

// But this will.

let c = [...a]; // before ES6 there was an even uglier way of doing this using "slice", but let's never talk about that

c[0] = 12345;

console.log(a[0], c[0]); // prints "99 12345"

There are different types of functions in modern-ish JavaScript: regular, async, and generator functions. However, typeof returns "function" for all of them. One way (the only?) to differentiate between them is by using toString.call(myFunc).

Due to a bug in the original C code for the implementation of Javascript, this awesome thing happens:

const a = null;

console.log(typeof a); // prints "object"

Here's yet another one of Javascript's endless oddities: the switch statement uses strict comparison (think === instead of ==).

There's a bunch of Web APIs that are pretty usable today:

Detect page visibility
Detect online state
Vibration
Detect orientation

...and more! clipboard, ambient light detection, battery status, etc.

You can run timers on the console object, like this:

console.time('a');

costlyOperation();

console.timeEnd('a'); // outputs how much time has passed since the call above

There's a nice utility that you can use to kill a process based on the port it's listening to, so for example if you have a server listening on port 80, you can run $ fkill :80 and it will murder it.

Only problem, it's made using Javascript, but if you're OK with having Node and plain-text JS files in your bin folder, you can install it with $ npm install --global fkill-cli.

Javascript's Date object has a bunch of methods to format dates as strings, including toLocaleDateString(). This method has options for controlling the format, which include language but also more granular options such as "use 2-digit years" or "show long weekday names".

Here are some examples (tests were done in Chrome 64):

In order to use a variable as an object's key, ES6 introduced something called "computed property names", where you add brackets to your variable and it gets replaced with its value.