12 Essential JavaScript Tips

In this article, I will share with you 12 tips about JavaScript. These tips may help you solve some problems in your actual work.

Related learning recommendations: javascript video tutorial

Use !!operator to convert Boolean value

Sometimes we need to check whether a variable exists or check whether the value has a valid value. If it exists, return the true value. In order to do such verification, we can use the !! operator, which is very convenient and simple. For variables, you can use !!variable for detection, as long as the value of the variable is: 0, null, " ", undefined or NaN will return false, otherwise true will be returned. 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, then account.hasMoney returns The value is true.

Use Convert strings to numbers

This technique is very useful. It is very simple and can convert string data into numbers, but it is only suitable for characters. 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 Numbers:

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

and conditionals

If you have a piece of code like this:

if (conected) {
    login();
}

You can also abbreviate the variables and use && and Functions connected together, like the example above, 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 ||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 parameter. If the value returned by the first parameter is false, then the second value will be considered a default value. Such as 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 in the looparray.length

This technique is very simple. This has an impact on performance when processing a large array loop. will be very big. Basically, everyone will write an array that synchronizes iteration 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 IE6 browser, but this function does not exist in IE6 browser, then It is very useful to use this operator to detect whether the function exists, as in the following example:

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

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

Get the last element in the array

Array.prototype.slice(begin,end) is used to get begin and end# Array elements between ##. 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]

10 elements, but you only want the first five elements, then 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]

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 the /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"

Array.concat() function:

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

然后这个函数并不适合用来合并两个大型的数组，因为其将消耗大量的内存来存储新创建的数组。在这种情况之个，可以使用Array.pus().apply(arr1,arr2)来替代创建一个新数组。这种方法不是用来创建一个新的数组，其只是将第一个第二个数组合并在一起，同时减少内存的使用：

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

将NodeList转换成数组

如果你运行document.querySelectorAll(“p”)函数时，它可能返回DOM元素的数组，也就是NodeList对象。但这个对象不具有数组的函数功能，比如sort()、reduce()、map()、filter()等。为了让这些原生的数组函数功能也能用于其上面，需要将节点列表转换成数组。可以使用[].slice.call(elements)来实现：

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

数组元素的洗牌

对于数组元素的洗牌，不需要使用任何外部的库，比如Lodash，只要这样做：

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

总结

现在你学会了些有用的JavaScript小技巧。希望这些小技巧能在工作中帮助你解决一些麻烦，或者说这篇文章对你有所帮助。如果你有一些优秀的JavaScript小技巧，欢迎在评论中与我们一起分享。

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