12 JavaScript Tips

Use the !! operator to convert a Boolean value

Sometimes we need to check whether a variable exists or check whether the value has a valid value, and return a true value if it exists. In order to do such verification, we can use the !! operator, which is very convenient and simple. You can use !!variable to detect variables. As long as the value of the variable is: 0, null, " ", undefined or NaN, it will return false, otherwise it will return true. For example, the following example:

function Account(cash) {
    this.cash = cash;
    this.hasMoney = !!cash;
}
var account = new Account(100.50);
console.log(account.cash); // 100.50
console.log(account.hasMoney); // true
var emptyAccount = new Account(0);
console.log(emptyAccount.cash); // 0
console.log(emptyAccount.hasMoney); // false

In this example, as long as the value of account.cash is greater than 0, the value returned by account.hasMoney is true.

Use + to convert strings into numbers

This technique is very useful, it is very simple, it can convert string data into numbers, but it is only suitable for String data, otherwise NaN will be returned, such as the following example:

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

This also applies to Date, in this case, it will return the timestamp number:

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

And conditional character

If you have a piece of code like this:

if (conected) {
    login();
}

You can also abbreviate variables and use && to connect them with functions, such as the above Example, can be abbreviated like this:

conected && login();

If some properties or functions exist in an object, you can also do this detection, as shown in the following code:

user && user.login();

Use The || operator

has the feature of default parameters in ES6. To emulate this feature in older browsers, use the || operator and pass the default value as the second argument. If the first parameter returns false, the second value will be considered a default value. Take the following example:

function User(name, age) {
    this.name = name || "Oliver Queen";
    this.age = age || 27;
}
var user1 = new User();
console.log(user1.name); // Oliver Queen
console.log(user1.age); // 27
var user2 = new User("Barry Allen", 25);
console.log(user2.name); // Barry Allen
console.log(user2.age); // 25

Cache array.length in a loop

This technique is very simple, but when processing a large array loop, the impact on performance will be very large. Basically, everyone will write an array that iterates synchronously like this:

for(var i = 0; i < array.length; i++) {
    console.log(array[i]);
}

If it is a small array, this is fine. If you are dealing with a large array, this code will iterate on each iteration. The array size will all be recalculated, which will cause some delays. In order to avoid this phenomenon, you can make a cache of array.length:

var length = array.length;
for(var i = 0; i < length; i++) {
    console.log(array[i]);
}

You can also write it like this:

for(var i = 0, length = array.length; i < length; i++) {
    console.log(array[i]);
}

Detect the attributes in the object

This little trick is useful when you need to detect the existence of some properties and avoid running undefined functions or properties. You might also want to use this trick if you plan to customize some cross-browser code. For example, you want to use document.querySelector() to select an id and make it compatible with the IE6 browser, but this function does not exist in the IE6 browser, so it is difficult to use this operator to detect whether the function exists. Very useful, as in the following example:

if ('querySelector' in document) {
    document.querySelector("#id");
} else {
    document.getElementById("id");
}

In this example, if the querySelector function does not exist in the document, then docuemnt.getElementById("id") will be called.

Get the last element in the array

Array.prototype.slice(begin,end) is used to get the array elements between begin and end. If you do not set the end parameter, the default length value of the array will be used as the end value. But some students may not know that this function can also accept negative values ​​as parameters. If you set a negative value as the value of begin, then you can get the last element of the array. For example:

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]

Array truncation

This tip is mainly used to lock the size of the array. It is very useful if it is used to delete some elements in the array. . For example, if your array has 10 elements, but you only want the first five elements, you can truncate the array by array.length=5. Such as the following 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]

Replace all

The String.replace() function allows you to use strings or regular expressions to replace strings. This function itself only Replace the first occurrence of the string, but you can use /g in the regular expression to simulate the replaceAll() function:

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

Merge arrays

If If you want to merge two arrays, generally you will 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];

Then this function is not suitable for merging two large arrays because it will consume a lot of memory. to store the newly created array. In this case, you can use Array.pus().apply(arr1, arr2) instead to create a new array. This method is not used to create a new array, it just merges the first and second arrays together while reducing 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];

Convert NodeList into an array

If you run the document.querySelectorAll("p") function, it may return an array of DOM elements, which is a NodeList object. But this object does not have the functions of an array, such as sort(), reduce(), map(), filter(), etc. In order for these native array functions to be used on it, the node list needs to be converted into an array. You can use [].slice.call(elements) to achieve:

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

Shuffling of array elements

For shuffling of array elements, there is no need to use any external For libraries, such as Lodash, just do this:

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

Summary

Now you have learned some useful JavaScript tricks. I hope these tips can help you solve some troubles at work, or that this article has been helpful to you. If you have some great JavaScript tips, please share them with us in the comments.