21个超实用的 JavaScript 技巧与实例

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If you are doing lot of JavaScript programming, you might find below list of code snippets very useful. Keep it handy (bookmark it) and save it for future reference.

Here are 20 very useful JavaScript tips and tricks for you.

Disclaimer: Not all these snippet are written by me. Some of them are collected from other sources on Internet.

1. Converting JavaScript Array to CSV

First one goes like this. You have a javascript array of strings (or numbers) and you want to convert it to comma separated values (csv). We’ll below is the code snippet:
Reference: Array to CSV in JavaScript

var fruits = ['apple', 'peaches', 'oranges', 'mangoes'];

var str = fruits.valueOf();

//print str: apple,peaches,oranges,mangoes

The valueOf() method will convert an array in javascript to a comma separated string.

Now what if you want to use pipe (|) as delimeter instead of comma. You can convert a js array into a delimeted string using join() method. See below:

var fruits = ['apple', 'peaches', 'oranges', 'mangoes'];

var str = fruits.join("|");

//print str: apple|peaches|oranges|mangoes

The join() method will convert the array into a pipe separated string.

2. Convert CSV to Array in JavaScript

Now what if you want to convert a comma seprated string into a JavaScript array? We’ll there is a method for that too. You can use split() method to split a string using any token (for instance comma) and create an array.

Reference: Array to CSV in JavaScript

In below example we split string str on comma (,) and create a fruitsArray array.

var str = "apple, peaches, oranges, mangoes";

var fruitsArray = str.split(",");

//print fruitsArray[0]: apple

3. Remove Array element by Index

You have an array. You want to remove a perticular element from array based on its index. We’ll you can do so using splice() method. This method can add as well as removes element from an array. We will stick to its removal usage.
Reference: Remove element by array index

function removeByIndex(arr, index) {

    arr.splice(index, 1);
}
 
test = new Array();

test[0] = 'Apple';

test[1] = 'Ball';

test[2] = 'Cat';

test[3] = 'Dog';
 
alert("Array before removing elements: "+test);
 
removeByIndex(test, 2);
 
alert("Array after removing elements: "+test);

4. Remove Array element by Value

This one is very useful. You have an array and you want to remove an element based on its value (instead of index).
Reference: Remove element by array index

function removeByValue(arr, val) {

    for(var i=0; i<arr.length; i++) {

        if(arr[i] == val) {

            arr.splice(i, 1);

            break;

        }

    }
}
 
var somearray = ["mon", "tue", "wed", "thur"]
 
removeByValue(somearray, "tue");
 
//somearray will now have "mon", "wed", "thur"

See how in above code we defined a method removeByValue that takes serves the purpose. In JavaScript you can define new functions to classes at runtime (although this is discourage) usingprototypes.

In below code snippet, we create a new method removeByValue() within Array class. So now you can call this method as any other arrays method.

Array.prototype.removeByValue = function(val) {

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

        if(this[i] == val) {

            this.splice(i, 1);

            break;

        }

    }
}
//..
 
var somearray = ["mon", "tue", "wed", "thur"]
 
somearray.removeByValue("tue");
 
//somearray will now have "mon", "wed", "thur"

5. Calling JavaScript function from String

Sometime you want to call a Javascript method at runtime whos name you know. Let say there is a method “foo()” which you want to call at runtime. Below is a small JS snippet that helps you calling a method just by its name.

Reference: Call Function as String

var strFun = "someFunction"; //Name of the function to be called

var strParam = "this is the parameter"; //Parameters to be passed in function

//Create the function

var fn = window[strFun];

//Call the function
fn(strParam);

6. Generate Random number from 1 to N

Below little snippet helps you in generating random number between 1 to N. Might come handy for your next JS game.

//random number from 1 to N

var random = Math.floor(Math.random() * N + 1);
 
//random number from 1 to 10

var random = Math.floor(Math.random() * 10 + 1);
 
//random number from 1 to 100

var random = Math.floor(Math.random() * 100 + 1);

7. Capture browser close button event or exiting the page in JavaScript

This is a common use case. You want to capture the browsers close event so that you can alert user if there is any unsaved data on webpage that should be saved. Below javascript snippet will help you in that.

Reference: Capture Browser Close Event

<script language="javascript">

function fnUnloadHandler() {

       // Add your code here

       alert("Unload event.. Do something to invalidate users session..");
}
</script>

<body onbeforeunload="fnUnloadHandler()">

    <!-- Your page content -->
</body>

8. Warn user if Back is pressed

This is same as above. Difference is instead of capturing close event here we capture back button event. So that we know if user is moving awaw from this webpage.
Reference: Capture Browser Back Button

window.onbeforeunload = function() { 

    return "You work will be lost."; 
};

9. Check if Form is Dirty

A common usecase. You need to check if user changed anything in an HTML form. Below functionformIsDirty returns a boolean value true or false depending on weather user modified something within the form.

/**

 * Determines if a form is dirty by comparing the current value of each element

 * with its default value.

 *

 * @param {Form} form the form to be checked.

 * @return {Boolean} <code>true</code> if the form is dirty, <code>false</code>

 *                   otherwise.

 */

function formIsDirty(form) {

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

    var element = form.elements[i];

    var type = element.type;

    if (type == "checkbox" || type == "radio") {

      if (element.checked != element.defaultChecked) {

        return true;

      }

    }

    else if (type == "hidden" || type == "password" ||

             type == "text" || type == "textarea") {

      if (element.value != element.defaultValue) {

        return true;

      }

    }

    else if (type == "select-one" || type == "select-multiple") {

      for (var j = 0; j < element.options.length; j++) {

        if (element.options[j].selected !=

            element.options[j].defaultSelected) {

          return true;

        }

      }

    }

  }

  return false;
}

window.onbeforeunload = function(e) {

  e = e || window.event;  

  if (formIsDirty(document.forms["someForm"])) {

    // For IE and Firefox

    if (e) {

      e.returnValue = "You have unsaved changes.";

    }

    // For Safari

    return "You have unsaved changes.";

  }
};

10. Disable Back button using JavaScript

This one is tricky. You want to disable the browsers back button (Dont ask me why!!). Below code snippet will let you do this. The only catch here is that you need to put this code in page where you dont want user to come back. See below reference for more details.
Reference: Disable Browsers Back Button

<SCRIPT type="text/javascript">

    window.history.forward();

    function noBack() { window.history.forward(); }
</SCRIPT>
</HEAD>

<BODY onload="noBack();"

    onpageshow="if (event.persisted) noBack();" onunload="">

11. Deleting Multiple Values From Listbox in JavaScript

You have a SELECT box. User can select multiple OPTIONS from this SELECT box and remove them. Below Javascript function selectBoxRemove let you do this. Just pass ID of SELECT object you want to remove OPTIONS in.

Reference: Delete Multiple Options in Select

function selectBoxRemove(sourceID) {

    //get the listbox object from id.

    var src = document.getElementById(sourceID);

    //iterate through each option of the listbox

    for(var count= src.options.length-1; count >= 0; count--) {

         //if the option is selected, delete the option

        if(src.options[count].selected == true) {

                try {

                         src.remove(count, null);

                 } catch(error) {

                         src.remove(count);

                }

        }

    }
}

12. Listbox Select All/Deselect All using JavaScript

You can use below JS snippet to select/deselect all the OPTIONS from a SELECT box. Just pass the ID of select box you want to perform this operation on and also a boolean value isSelect specifying what operation you want to perform.
Reference: Select All/None using Javascript

function listboxSelectDeselect(listID, isSelect) {

    var listbox = document.getElementById(listID);

    for(var count=0; count < listbox.options.length; count++) {

            listbox.options[count].selected = isSelect;

    }
}

13. Listbox Move selected items Up / Down

This one is useful if you are playing a lot with multi options select box in your application. This function let you move select OPTIONS in a SELECT box to UP or DOWN. See below reference for more details.
Reference: Move Up/Down Selected Items in a Listbox

function listbox_move(listID, direction) {

    var listbox = document.getElementById(listID);

    var selIndex = listbox.selectedIndex;

    if(-1 == selIndex) {

        alert("Please select an option to move.");

        return;

    }

    var increment = -1;

    if(direction == 'up')

        increment = -1;

    else

        increment = 1;

    if((selIndex + increment) < 0 ||

        (selIndex + increment) > (listbox.options.length-1)) {

        return;

    }

    var selValue = listbox.options[selIndex].value;

    var selText = listbox.options[selIndex].text;

    listbox.options[selIndex].value = listbox.options[selIndex + increment].value

    listbox.options[selIndex].text = listbox.options[selIndex + increment].text

    listbox.options[selIndex + increment].value = selValue;

    listbox.options[selIndex + increment].text = selText;

    listbox.selectedIndex = selIndex + increment;
}
//..
//..
 
listbox_move('countryList', 'up'); //move up the selected option

listbox_move('countryList', 'down'); //move down the selected option

14. Listbox Move Left/Right Options

This javascript function lets you move selected OPTIONS between two SELECT boxes. Check below reference for details.
Reference: Move Options Left/Right between two SELECT boxes

function listbox_moveacross(sourceID, destID) {

    var src = document.getElementById(sourceID);

    var dest = document.getElementById(destID);

    for(var count=0; count < src.options.length; count++) {

        if(src.options[count].selected == true) {

                var option = src.options[count];

                var newOption = document.createElement("option");

                newOption.value = option.value;

                newOption.text = option.text;

                newOption.selected = true;

                try {

                         dest.add(newOption, null); //Standard

                         src.remove(count, null);

                 }catch(error) {

                         dest.add(newOption); // IE only

                         src.remove(count);

                 }

                count--;

        }

    }
}
//..
//..
 
listbox_moveacross('countryList', 'selectedCountryList');

15. Initialize JavaScript Array with series of numbers

Sometime you want to initialize a Javascript array with series of numbers. Below code snippet will let you achieve this. This will initialiaze array numbers with numbers 1 to 100.

var numbers = [];

for(var i=1; numbers.push(i++)<100;);
//numbers = [0,1,2,3 ... 100]

16. Rounding Numbers to ‘N’ Decimals

This one is quite useful. It will let you round off a number to ‘N’ decimal places. Here in below example we are rounding of a number to 2 decimal places.

var num = 2.443242342;

alert(num.toFixed(2)); // 2.44

Note that we use toFixed() method here. toFixed(n) provides n length after the decimal point; whereastoPrecision(x) provides x total length.

num = 500.2349;

result = num.toPrecision(4); // result will equal 500.2

17. Check if String contains another substring in JavaScript

Below code snippet (courtesy Stackoverflow) let you check if a given string contains another substring.

Reference: stackoverflow

if (!Array.prototype.indexOf) { 

    Array.prototype.indexOf = function(obj, start) {

         for (var i = (start || 0), j = this.length; i < j; i++) {

             if (this[i] === obj) { return i; }

         }

         return -1;

    }
}
 
if (!String.prototype.contains) {

    String.prototype.contains = function (arg) {

        return !!~this.indexOf(arg);

    };
}

The code will add two new methods to String, indexOf and contains. Once this is done, you can use contains method to check weather a substring is present in a given string.

var hay = "a quick brown fox jumps over lazy dog";
var needle = "jumps";
alert(hay.contains(needle));

18. Remove Duplicates from JavaScript Array

Aha!! You know this one comes quite handy. Just call removeDuplicates method and pass the array, it should remove all duplicate elements from the array.

function removeDuplicates(arr) {

    var temp = {};

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

        temp[arr[i]] = true;
 
    var r = [];

    for (var k in temp)

        r.push(k);

    return r;
}
 
//Usage

var fruits = ['apple', 'orange', 'peach', 'apple', 'strawberry', 'orange'];

var uniquefruits = removeDuplicates(fruits);
//print uniquefruits ['apple', 'orange', 'peach', 'strawberry'];

19. Trim a String in JavaScript

Below code will add a method trim() to String which can be used to remove spaces from both sides of a string.

if (!String.prototype.trim) {

   String.prototype.trim=function() {

    return this.replace(/^\s+|\s+$/g, '');

   };
}
 
//usage

var str = "  some string    ";
str.trim();
//print str = "some string"

20. Redirect webpage in JavaScript

This javascript code should perform http redirect on a given URL.

window.location.href = "http://viralpatel.net";

21. Encode a URL in JavaScript

Whenever you are making an http request and passing some parameters. You should encode the URL string. Use encodeURIComponent function for encoding a url parameter.
Reference: w3scools

var myOtherUrl = 

       "http://example.com/index.html?url=" + encodeURIComponent(myUrl);

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2013-05-24 13:59
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