Why prompt returns sometimes null as string [duplicate]

Answer 1

@RahulArora has basically provided the right answer, but to provide a bit more illumination about what is happening:

Browsers have a built-in global variable called name which stores the name of the window. If you assign a value to it, the value you assign will be converted to a string. This has nothing to do with prompt(); you can observe it like this:

var name = null;
console.log(typeof name);  // "string"
console.log(name.length);  // 4

The moral of the story: Avoid using global variables unless you absolutely have to, and if you do, make sure they're not already being used for something else!

Answer 2

One line answer to your question is that, it returns null when user has escaped the prompt box by clicking cancel or pressing escape.

For further clarification of concepts read this :-

This is formal defiinition of function:

result = window.prompt(message, default);

result : is a string containing the text entered by the user, or null.

message : is a string of text to display to the user. This parameter is optional and can be omitted if there is nothing to show in the prompt window.

default : is a string containing the default value displayed in the text input field. It is an optional parameter. Note that in Internet Explorer 7 and 8, if you do not provide this parameter, the string "undefined" is the default value. (Above was from Mozilla's definition of prompt())

Now we Conclude that:

prompt() returns either null or "string" which can include ""(empty string).

Now We have three states to inspect:

null : The user has clicked Cancel or pressed Esc.

""(empty string) : The user clicked OK or pressed Enter with no text input

"string" : The user entered some text.

hope it helps...!

Answer 3

Here is your answer

The keyword 'name' causes problems when used in global scope in javascript

If even in your first case, you replace 'name' with 'fname' or some other variable name it will stop giving null.

<script type="text/javascript">
    var fname = prompt("Enter your name:", "");
    if (!fname) {
        fname = "unknown";      
    }
    var ans =("<h2>" + "Hello, "+ fname + "!"+"</h2>");     
    document.write(ans);        
</script>

In the second case variable 'name' is locally created inside the function and it does not conflict with window.name. Hence it works properly