12 Extremely Useful Hacks for JavaScript

April 28th, 2016 | By Caio Ribeiro Pereira | 6 min read

In this post I will share 12 extremely useful hacks for JavaScript. These hacks reduce the code and will help you to run optimized code. So let’s start hacking!

1. Converting to boolean using !! operator

Sometimes we need to check if some variable exists or if it has a valid value, to consider them as true value. For do this kind of validation, you can use the !!(Double negation operator) a simple !!variable, which will automatically convert any kind of data to boolean and this variable will return false only if it has some of these values: 0, null, "", undefined or NaN, otherwise it will return true. To understand it in practice, take a look this simple example:

function Account(cash) { = cash;
    this.hasMoney = !!cash;
var account = new Account(100.50);
console.log(; // 100.50
console.log(account.hasMoney); // true

var emptyAccount = new Account(0);
console.log(; // 0
console.log(emptyAccount.hasMoney); // false

In this case, if an value is greater than zero, the account.hasMoney will be true.

2. Converting to number using + operator

This magic is awesome! And it’s very simple to do, but it only works with string numbers, otherwise, it will return NaN(Not a Number). Have a look on this example:

function toNumber(strNumber) {
    return +strNumber;
console.log(toNumber("1234")); // 1234
console.log(toNumber("ACB")); // NaN

This magic will work with Date too and, in this case, it will return the timestamp number:

console.log(+new Date()) // 1461288164385

3. Short-circuits conditionals

If you see a similar code:

if (conected) {

You can shorten it by using the combination of a variable (which will be verified) and a function using the && (AND operator) between both. For example, the previous code can become smaller in one line:

conected && login();

You can do the same to check if some attribute or function exists in the object. Similar to the below code:

user && user.login();

4. Default values using || operator

Today in ES6 there is the default argument feature. In order to simulate this feature in old browsers you can use the || (OR operator) by including the default value as a second parameter to be used. If the first parameter returns false the second one will be used as a default value. See this example:

function User(name, age) { = name || "Oliver Queen";
    this.age = age || 27;
var user1 = new User();
console.log(; // Oliver Queen
console.log(user1.age); // 27

var user2 = new User("Barry Allen", 25);
console.log(; // Barry Allen
console.log(user2.age); // 25

5. Caching the array.length in the loop

This tip is very simple and causes a huge impact on the performance when processing large arrays during a loop. Basically, almost everybody writes this synchronously to iterate an array:

for (var i = 0; i < array.length; i++) {

If you work with smaller arrays – it’s fine, but if you process large arrays, this code will recalculate the size of array in every iteration of this loop and this will cause a bit of delays. To avoid it, you can cache the array.length in a variable to use it instead of invoking the array.length every time during the loop:

var length = array.length;
for (var i = 0; i < length; i++) {

To make it smaller, just write this code:

for (var i = 0, length = array.length; i < length; i++) {

6. Detecting properties in an object

This trick is very useful when you need to check if some attribute exists and it avoids running undefined functions or attributes. If you are planning to write cross-browser code, probably you will use this technique too. For example, let’s imagine that you need to write code that is compatible with the old Internet Explorer 6 and you want to use the document.querySelector(), to get some elements by their ids. However, in this browser, this function doesn’t exist, so to check the existence of this function you can use the in operator, see this example:

if ('querySelector' in document) {
} else {

In this case, if there is no querySelector function in the document object, we can use the document.getElementById() as a fallback.

7. Getting the last item in the array

The Array.prototype.slice(begin, end) has the power to cut arrays when you set the begin and end arguments. But if you don’t set the end argument, this function will automatically set the max value for the array. I think that few people know that this function can accept negative values, and if you set a negative number as begin argument you will get the last elements from the array:

var array = [1, 2, 3, 4, 5, 6];
console.log(array.slice(-1)); // [6]
console.log(array.slice(-2)); // [5,6]
console.log(array.slice(-3)); // [4,5,6]

8. Array truncation

This technique can lock the array’s size, this is very useful to delete some elements of the array based on the number of elements you want to set. For example, if you have an array with 10 elements, but you want to get only the first five elements, you can truncate the array, making it smaller by setting the array.length = 5. See this example:

var array = [1, 2, 3, 4, 5, 6];
console.log(array.length); // 6
array.length = 3;
console.log(array.length); // 3
console.log(array); // [1,2,3]

9. Replace all

The String.replace() function allows using String and Regex to replace strings, natively this function only replaces the first occurrence. But you can simulate a replaceAll() function by using the /g at the end of a Regex:

var string = "john john";
console.log(string.replace(/hn/, "ana")); // "joana john"
console.log(string.replace(/hn/g, "ana")); // "joana joana"

10. Merging arrays

If you need to merge two arrays you can use the Array.concat() function:

var array1 = [1, 2, 3];
var array2 = [4, 5, 6];
console.log(array1.concat(array2)); // [1,2,3,4,5,6];

However, this function is not the most suitable to merge large arrays because it will consume a lot of memory by creating a new array. In this case, you can use Array.push.apply(arr1, arr2) which instead creates a new array – it will merge the second array in the first one reducing the memory usage:

var array1 = [1, 2, 3];
var array2 = [4, 5, 6];
console.log(array1.push.apply(array1, array2)); // [1,2,3,4,5,6];

11. Converting NodeList to Arrays

If you run the document.querySelectorAll("p") function, it will probably return an array of DOM elements, the NodeList object. But this object doesn’t have all array’s functions, like: sort(), reduce(), map(), filter(). In order to enable these and many other native array’s functions you need to convert NodeList into Arrays. To run this technique just use this function: []

var elements = document.querySelectorAll("p"); // NodeList
var arrayElements = []; // Now the NodeList is an array
var arrayElements = Array.from(elements); // This is another way of converting NodeL

12. Shuffling array’s elements

To shuffle the array’s elements without using any external library like Lodash, just run this magic trick:

var list = [1, 2, 3];
console.log(list.sort(function() {
    return Math.random() - 0.5
})); // [2,1,3]


Now you've learned some useful JS hacks that are largely used to minify JavaScript code, and some of these tricks are used in many popular JS frameworks like Lodash, Underscore.js, and Strings.js, among others.

If you want to go deeper and learn more about how you can minify your code even more and protect it from prying eyes, talk to us.

I hope you enjoyed this post, and if you know of other JS hacks, please let us know!


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