A prompt box asks the user for some information and provides a response field for her
answer.
This code asks the user the question "Your species?" and provides a default answer in the
response field, "human". She can change the response. Whether she leaves the default response
as-is or changes it to something else, her response is assigned to the variable.
var spec = prompt("Your species?", "human");
Prompt code is like alert code, with two differences.
In a prompt, you need a way to capture the user's response. That means you need to start
by declaring a variable, followed by an equal sign.
In a prompt, you can specify a second string. This is the default response that appears in
the field when the prompt displays. If the user leaves the default response as-is and just
clicks OK, the default response is assigned to the variable. It's up to you whether you
include a default response.
As you might expect, you can assign the strings to variables, then specify the variables
instead of strings inside the parentheses.
var question = "Your species?";
var defaultAnswer = "human";
var spec = prompt(question, defaultAnswer);
The user's response is a text string. Even if the response is a number, it comes back as a
string. For example, consider this code.
var numberOfCats = prompt("How many cats?");
var tooManyCats = numberOfCats + 1;
Since you're asking for a number, and the user is presumably entering one, you might
expect the math in the second statement to work. For example, if the user enters 3, the variable
tooManyCats should have a value of 4, you might think. But no such luck. All responses to
prompts come back as strings. When the string, "3", is linked with a plus to the number, 1,
JavaScript converts the 1 to a string and concatenates. So the value of tooManyCats winds up
being not 4 but "31". You'll learn how to solve this problem in a subsequent chaper.
If the user enters nothing and clicks OK, the variable is assigned an empty string: ""
If the user clicks Cancel, the variable is assigned a special value, null.
Some coders write window.prompt instead of, simply, prompt. This is a highly formal
but perfectly correct way to write it. Most coders prefer the short form. We'll stick to the
short form in this training.
In the example above, some coders would use single rather than double quotation marks.
This is legal, as long as it's a matching pair. But in a case like this, I'll ask you to use
double quotation marks.
