Once we know how to show output from JavaScript, let's have a quick look at two ways to receive input. Probably neither of these is use a lot, but they can be used easily to play around with the basics.


The fist one is called prompt. It will show a pop-up window with the text provided as the first parameter and with a textbox the user can fill in. When the user presses OK, the value in the text box will be returned by the prompt() function. Then, in this example we use the document.write method to update the html with the text.



var name = prompt("Your name:", "");
document.write("Hello ", name);


The textbox will be pre-filled with the content of the second parameter. This can be very useful if we would like to ask the user to edit some value. We can pre-fill the box with the old value.



var name = prompt("Please correct your e-mail address:", "foo@bar.co");
document.write("Your e-mail address is ", name);


In either case, if the user presses cancel or hits the ESC the prompt() function will return null.


The other pop-up is not really an input method. It allows the developer to ask a Yes/No question. Calling the confirm() function will show a pop-up window with the provided texts and with two buttons. If the user presses OK the confirm() function will return true, if the user presses cancel or hits the ESC key, the function will return false.

Of course in order for this to make more sense you'll have to understand what true and false really mean and what this if - else construct does. If you have programming background then you probably already understand the code, and even if you don't have programming background you might figure out.

That code can basically be translated to the following English sentence:

If confirm returned true, print "Hello World", otherwise print "OK, I won't print it."

Or even better:

If the user presses OK when we asked "Shall I print Hello World?", then print "Hello World", otherwise print "OK, I won't print it."



if (confirm("Shall I print Hello World?")) {
    document.write("Hello World");
} else {
    document.write("OK, I won't print it.");