JavaScript usage Tips

Use !!Operator Conversion Boolean value

Sometimes we need to check whether a variable exists or checks if the value has a valid value, and returns the value if it exists true . In order to do this verification, we can use the !! operator to achieve is very convenient and simple. For a variable to be used for !!variable detection, as long as the value of the variable is: 0 ,, null " " , undefined or NaN all will be returned false , the reverse is returned true .

Use +Convert a string into a number

You can convert the string data to a number, but it is only appropriate for string data, otherwise it will be returned NaN , such as the following example:

function Tonumber (strnumber) {    return +Strnumber;} Console.log (Tonumber (//  1234//  NaN

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

Console.log (+new//  1461288164385

and the conditional character

If you have a code like this:

if (conected) {    login ();}

You can also abbreviate variables and use them && together with functions, such as the example above, which can be abbreviated as follows:

conected && Login ();

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

User && user.login ();

Use ||Operator

This feature has default parameters in ES6. To emulate this feature in an older version of the browser, you can use the || operator and pass the default value as the second parameter. If the first parameter returns a value of false , then the second value will be considered a default value. As the following example:

functionUser (name, age) { This. Name = Name | | "Oliver Queen";  This. Age = Age | | 27;}varUser1 =NewUser (); Console.log (user1.name);//Oliver QueenConsole.log (User1.age);// -varUser2 =NewUser ("Barry Allen", 25); Console.log (User2.name); //Barry AllenConsole.log (User2.age);// -

Caching in Loops array.length

This is a very simple technique, which will have a very large effect on performance when dealing with a large array loop. Basically, you will write an array of such synchronous iterations:

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

If it is a small array, this is good, if you are dealing with a large array, this code will recalculate the size of the array in each iteration, which will cause some delay. To avoid this phenomenon, you can array.length do a cache:

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

You can also write in this way:

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

Detecting Properties in Objects

This trick is useful when you need to detect whether some properties exist and avoid running undefined functions or properties. If you're going to set some cross-compatible browser code, you might as well use this little trick. For example, if you want document.querySelector() to use to select one and id make it compatible with the IE6 browser, but this function does not exist in the IE6 browser, it is very useful to use this operator to detect the existence of this function, as in the following example:

if inch document) {    Document.queryselector ("#id"else  {    document.getElementById ("id") ;}

In this example, if document there is no querySelector function, then it is called docuemnt.getElementById("id") .

Gets the last element in the array

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

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

Array truncation

This trick is primarily used to lock the size of an array, which is useful if you are using to delete some of the elements in the array. For example, your array has 10 an element, but you want only the first five elements, then you can truncate the array.length=5 array by. As the following example:

var array = [1,2,3,4,5,6//  6array.length = 3//  3//  [A]

Merging arrays

If you are merging two arrays, you will normally use the Array.concat() function:

var = [Array1]; var array2 = [4,5,6//  [1,2,3,4,5,6];

Then this function is not suitable for merging two large arrays, because it consumes a lot of memory to store the newly created array. In this case, you can use an Array.pus().apply(arr1,arr2) alternative to create a new array. This method is not used to create a new array, it simply combines the first second number together and reduces the use of memory:

var = [Array1]; var array2 = [4,5,6//  [1,2,3,4,5,6];

Shuffle of array elements

For the shuffling of array elements, you do not need to use any external libraries, such as Lodash, as long as you do this:

var list = [];console.log (List.sort) (function//  [2,1,3]

JavaScript usage Tips