The way to add javascript code to the client is to place it in the URL after the pseudo-protocol specifier javascript:. This special protocol type declares that the body of the URL is arbitrary javascript code, which is run by the javascript interpreter. If the javascript code in the javascript:URL contains multiple statements, the statements must be separated using a semicolon. Such a URL looks like this:
javascript:var now = new Date(); "<h1>The time is:</h1>" + now;
When the browser loads such a URL, it will execute the javascript code contained in the URL and display the string value of the last javascript statement as the content of the new document. This string value can contain HTML tags and is formatted to display exactly the same as other documents loaded into the browser.
The javascript URL can also contain javascript statements that only perform actions but do not return values. For example:
javascript:alert("hello world!")
When such a URL is loaded, the browser only executes the javascript code inside it, but since there is no value displayed as a new document, it does not change the currently displayed document.
Usually we want to use javascript:URL to execute some javascript code that does not change the currently displayed document. To do this, you must make sure that the last statement in the URL does not return a value. One way is to explicitly specify the return value as underfined using the void operator, just use the statement void 0; at the end of the javascript:URL. For example: The following URL will open a new empty browser window without changing the content of the current window:
javascript:("about:blank"); void 0;
If this URL does not have a void operator, the return value of the () method will be converted into a string and displayed, and the current window will be overwritten by the document shown below.