Introducing Javascript Tips

New year, new project. A JS tip per day!

With great excitement, I introduce these short and useful daily Javascript tips that will allow you to improve your code writing. With less than 2 minutes each day, you will be able to read about performance, frameworks, conventions, hacks, interview questions and all the items that the future of this awesome language holds for us.

At midday, no matter if it is a weekend or a holiday, a tip will be posted and tweeted.

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Please feel free to send us a PR with your own Javascript tip to be published here. Any improvements or suggestions are more than welcome! Click to see the instructions

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Tips list

#08 - Converting a Node List to an Array

2016-01-08 by @Tevko

The querySelectorAll method returns an array-like object called a node list. These data structures are referred to as "Array-like", because they appear as an array, but can not be used with array methods like map and foreach. Here's a quick, safe, and reusable way to convert a node list into an Array of DOM elements:

const nodelist = document.querySelectorAll('div');
const nodelistToArray = Array.apply(null, nodelist);

//later on ..

nodelistToArray.forEach(...);
nodelistToArray.map(...);
nodelistToArray.slice(...);

//etc...

The apply method is used to pass an array of arguments to a function with a given this value. MDN states that apply will take an array like object, which is exactly what querySelectorAll returns. Since we don't need to specify a value for this in the context of the function, we pass in null or 0. The result is an actual array of DOM elements which contains all of the available array methods.

#07 - "use strict" and get lazy

2016-01-07 by @nainslie

Strict-mode JavaScript makes it easier for the developer to write "secure" JavaScript.

By default, JavaScript allows the programmer to be pretty careless, for example, by not requiring us to declare our variables with "var" when we first introduce them. While this may seem like a convenience to the unseasoned developer, it's also the source of many errors when a variable name is misspelled or accidentally referred to out of its scope.

Programmers like to make the computer do the boring stuff for us, and automatically check our work for mistakes. That's what the JavaScript "use strict" directive allows us to do, by turning our mistakes into JavaScript errors.

We add this directive either by adding it at the top of a js file:

// Whole-script strict mode syntax
"use strict";
var v = "Hi!  I'm a strict mode script!";

or inside a function:

function f()
{
  // Function-level strict mode syntax
  'use strict';
  function nested() { return "And so am I!"; }
  return "Hi!  I'm a strict mode function!  " + nested();
}
function f2() { return "I'm not strict."; }

By including this directive in a JavaScript file or function, we will direct the JavaScript engine to execute in strict mode which disables a bunch of behaviors that are usually undesirable in larger JavaScript projects. Among other things, strict mode changes the following behaviors:

• Variables can only be introduced when they are preceded with "var"
• Attempting to write to readonly properties generates a noisy error
• You have to call constructors with the "new" keyword
• "this" is not implicitly bound to the global object
• Very limited use of eval() allowed
• Protects you from using reserved words or future reserved words as variable names

Strict mode is great for new projects, but can be challenging to introduce into older projects that don't already use it in most places. It also can be problematic if your build chain concatenates all your js files into one big file, as this may cause all files to execute in strict mode.

It is not a statement, but a literal expression, ignored by earlier versions of JavaScript. Strict mode is supported in:

• Internet Explorer from version 10.
• Firefox from version 4.
• Chrome from version 13.
• Safari from version 5.1.
• Opera from version 12.

See MDN for a fuller description of strict mode.

#06 - Writing a single method for arrays or single elements

2016-01-06 by @mattfxyz

Rather than writing separate methods to handle an array and a single element parameter, write your functions so they can handle both. This is similar to how some of jQuery's functions work (css will modify everything matched by the selector).

You just have to concat everything into an array first. Array.concat will accept an array or a single element.

function printUpperCase(words) {
  var elements = [].concat(words);
  for (var i = 0; i < elements.length; i++) {
    console.log(elements[i].toUpperCase());
  }
}

printUpperCase is now ready to accept a single node or an array of nodes as its parameter.

printUpperCase("cactus");
// => CACTUS
printUpperCase(["cactus", "bear", "potato"]);
// => CACTUS
//  BEAR
//  POTATO

#05 - Differences between undefined and null

2016-01-05 by @loverajoel

• undefined means a variable has not been declared, or has been declared but has not yet been assigned a value

• null is an assignment value that means "no value"

• Javascript sets unassigned variables with a default value of undefined

• Javascript never sets a value to null. It is used by programmers to indicate that a var has no value.

• undefined is not valid in JSON while null is

• undefined typeof is undefined

• null typeof is an object

• Both are primitives

• Both are falsy (Boolean(undefined) // false, Boolean(null) // false)

• You can know if a variable is undefined

  typeof variable === "undefined"

- You can check if a variable is [null](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/null)

  ```javascript
  variable === null

• The equality operator considers them equal, but the identity doesn't

  null == undefined // true
  
  null === undefined // false

## #04 - Sorting strings with accented characters

> 2016-01-04 by [@loverajoel](https://twitter.com/loverajoel)

Javascript has a native method **[sort](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort)** that allows sorting arrays. Doing a simple `array.sort()` will treat each array entry as a string and sort it alphabetically. Also you can provide your [own custom sorting](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort#Parameters) function.

```javascript
['Shanghai', 'New York', 'Mumbai', 'Buenos Aires'].sort();
// ["Buenos Aires", "Mumbai", "New York", "Shanghai"]

But when you try order an array of non ASCII characters like this ['é', 'a', 'ú', 'c'], you will obtain a strange result ['c', 'e', 'á', 'ú']. That happens because sort works only with english language.

See the next example:

// Spanish
['único','árbol', 'cosas', 'fútbol'].sort();
// ["cosas", "fútbol", "árbol", "único"] // bad order

// German
['Woche', 'wöchentlich', 'wäre', 'Wann'].sort();
// ["Wann", "Woche", "wäre", "wöchentlich"] // bad order

Fortunately, there are two ways to overcome this behavior localeCompare and Intl.Collator provided by ECMAScript Internationalization API.

Both methods have their own custom parameters in order to configure it to work adequately.

Using localeCompare()

['único','árbol', 'cosas', 'fútbol'].sort(function (a, b) {
  return a.localeCompare(b);
});
// ["árbol", "cosas", "fútbol", "único"]

['Woche', 'wöchentlich', 'wäre', 'Wann'].sort(function (a, b) {
  return a.localeCompare(b);
});
// ["Wann", "wäre", "Woche", "wöchentlich"]

Using Intl.Collator()

['único','árbol', 'cosas', 'fútbol'].sort(Intl.Collator().compare);
// ["árbol", "cosas", "fútbol", "único"]

['Woche', 'wöchentlich', 'wäre', 'Wann'].sort(Intl.Collator().compare);
// ["Wann", "wäre", "Woche", "wöchentlich"]

• For each method you can customize the location.
• According to Firefox Intl.Collator is faster when comparing large numbers of strings.

So when you are working with arrays of strings in a language other than English, remember to use this method to avoid unexpected sorting.

#03 - Improve Nested Conditionals

2016-01-03 by AlbertoFuente

How can we improve and make more efficient nested if statement in javascript.

if (color) {
  if (color === 'black') {
    printBlackBackground();
  } else if (color === 'red') {
    printRedBackground();
  } else if (color === 'blue') {
    printBlueBackground();
  } else if (color === 'green') {
    printGreenBackground();
  } else {
    printYellowBackground();
  }
}

One way to improve the nested if statement would be using the switch statement. Although it is less verbose and is more ordered, It's not recommended to use it because it's so difficult to debug errors, here's why.

switch(color) {
  case 'black':
    printBlackBackground();
    break;
  case 'red':
    printRedBackground();
    break;
  case 'blue':
    printBlueBackground();
    break;
  case 'green':
    printGreenBackground();
    break;
  default:
    printYellowBackground();
}

But what if we have a conditional with several checks in each statement? In this case, if we like to do less verbose and more ordered, we can use the conditional switch. If we pass true as parameter to the switch statement, It allows us to put a conditional in each case.

switch(true) {
  case (typeof color === 'string' && color === 'black'):
    printBlackBackground();
    break;
  case (typeof color === 'string' && color === 'red'):
    printRedBackground();
    break;
  case (typeof color === 'string' && color === 'blue'):
    printBlueBackground();
    break;
  case (typeof color === 'string' && color === 'green'):
    printGreenBackground();
    break;
  case (typeof color === 'string' && color === 'yellow'):
    printYellowBackground();
    break;
}

But we must always avoid having several checks in every condition, avoiding use of switch as far as possible and take into account that the most efficient way to do this is through an object.

var colorObj = {
  'black': printBlackBackground,
  'red': printRedBackground,
  'blue': printBlueBackground,
  'green': printGreenBackground,
  'yellow': printYellowBackground
};

if (color && colorObj.hasOwnProperty(color)) {
  colorObj[color]();
}

Here you can find more information about this.

#02 - ReactJs - Keys in children components are important

2016-01-02 by @loverajoel

The key is an attribute that you must pass to all components created dynamically from an array. It's unique and constant id that React use for identify each component in the DOM and know that it's a different component and not the same one. Using keys will ensure that the child component is preserved and not recreated and prevent that weird things happens.

Key is not really about performance, it's more about identity (which in turn leads to better performance). randomly assigned and changing values are not identity Paul O’Shannessy

• Use an exisiting unique value of the object.

• Define the keys in the parent components, not in child components

   //bad
   ...
   render() {
   	<div key={{item.key}}>{{item.name}}</div>
   }
   ...
  
   //good
   <MyComponent key={{item.key}}/>

• Using array index is a bad practice.

• random() will not work

   //bad
   <MyComponent key={{Math.random()}}/>

• You can create your own unique id, be sure that the method be fast and attach it to your object.

• When the amount of child are big or involve expensive components, use keys has performance improvements.

• You must provide the key attribute for all children of ReactCSSTransitionGroup.

#1 - AngularJs: $digest vs $apply

2016-01-01 by @loverajoel

One of the most appreciated features of AngularJs is the two way data binding. In order to make this work AngularJs evaluates the changes between the model and the view through cycles($digest). You need to understand this concept in order to understand how the framework works under the hood.

Angular evaluates each watcher whenever one event is fired, this is the known $digest cycle. Sometimes you have to force to run a new cycle manually and you must choose the correct option because this phase is one of the most influential in terms of performance.

$apply

This core method lets you to start the digestion cycle explicitly, that means that all watchers are checked, the entire application starts the $digest loop. Internally after execute an optional function parameter, call internally to $rootScope.$digest();.

$digest

In this case the $digest method starts the $digest cycle for the current scope and its children. You should notice that the parents scopes will not be checked and not be affected.

Recommendations

• Use $apply or $digest only when browser DOM events have triggered outside of AngularJS.

• Pass a function expression to $apply, this have a error handling mechanism and allow integrate changes in the digest cycle

   $scope.$apply(() => {
   	$scope.tip = 'Javascript Tip';
   });

• If only needs update the current scope or its children use $digest, and prevent a new digest cycle for the whole application. The performance benefit it's self evident

• $apply() is hard process for the machine and can lead to performance issues when having a lot of binding.

• If you are using >AngularJS 1.2.X, use $evalAsync is a core method that will evaluate the expression during the current cycle or the next. This can improve your application's performance.

#0 - Insert item inside an Array

2015-12-29

Inserting an item into an existing array is a daily common task. You can add elements to the end of an array using push, to the beginning using unshift, or the middle using splice.

But those are known methods, doesn't mean there isn't a more performant way, here we go...

Adding an element at the end of the array is easy with push(), but there is a way more performant.

var arr = [1,2,3,4,5];

arr.push(6);
arr[arr.length] = 6; // 43% faster in Chrome 47.0.2526.106 on Mac OS X 10.11.1

Both methods modify the original array. Don't believe me? Check the jsperf

Now we are trying to add an item to the beginning of the array

var arr = [1,2,3,4,5];

arr.unshift(0);
[0].concat(arr); // 98% faster in Chrome 47.0.2526.106 on Mac OS X 10.11.1

Here is a little bit detail, unshift edits the original array, concat returns a new array. jsperf

Adding items at the middle of an array is easy with splice and is the most performant way to do it.

var items = ['one', 'two', 'three', 'four'];
items.splice(items.length / 2, 0, 'hello');

I tried to run these tests in various Browsers and OS and the results were similar. I hope these tips will be useful for you and encourage to perform your own tests!

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