Develop Reference

!variable not working in javascript

I have a javascript: bookmarklet with the code
javascript:document.body.contentEditable = !document.body.contentEditable;
which should switch on and off an "editor" for the page (just for pranks on friends and such). But it does not acheive the desired outcome, nothing happens when I click the bookmark. Opening up the Javascript Console, I see that:
document.body.contentEditable
"false"
!document.body.contentEditable
false
Previously, I used javascript:document.body.contentEditable = true; and this makes the page editable but I cannot turn it off.

Like you probably noticed in the JavaScript Console, document.body.contentEditable is a String, not a Boolean. You can do this instead:
document.body.contentEditable = !(document.body.contentEditable == "true");
or just
document.body.contentEditable = document.body.contentEditable != "true";
The HTMLElement.contentEditable property is used to indicate whether
or not the element is editable. This enumerated attribute can have the
following values:
"true" indicates that the element is contenteditable.
"false" indicates that the element cannot be edited.
"inherit" indicates that the element inherits its parent's editable status.
https://developer.mozilla.org/en/docs/Web/API/HTMLElement/contentEditable

document.body.contentEditable is a string value and JavaScript considers non-empty strings to be truthy.
!"" == true
!"a" == false

The value of contentEditable is a string, not a boolean. It can have the values "true", "false" or "inherit" (therefore it can't be a simple boolean). A boolean inversion won't work. You need to explicitly assign one of these three strings.

Is contentEditable a text input? So you need to parse the texto into boolean:
javascript:document.body.contentEditable = !JSON.parse(document.body.contentEditable);

deceze is correct here, I (somewhat stupidly) thought that because you do
document.body.contentEditable = true;
you could also do
!document.body.contentEditable;
but this is not correct.
In the end, I decided to use the property that is actually a boolean, isContentEditable, like so:
document.body.contentEditable = !document.body.isContentEditable;

Javascript regex matching on a time

I want to see if the string I have is in the form of HH:MM:SS.
Here is what I have so far:
d = '00:01:01'
d.match(/\d{2}:\d{2}:\d{2}/)
["00:01:02"]
Is there a way to just get a True/False, instead of an array?

Use .test method of Regexp object.
/\d{2}:\d{2}:\d{2}/.test(d)
// true

Perhaps, you can use regex.test(str) but it's also possible using match because on success, match returns an array which is a truthy value and on failure, match returns null which is a falsy value. Check this to understand truthy and falsy.
So, if you use
if(d.match(/\d{2}:\d{2}:\d{2}/)) {
// true
}
else {
// false
}
This will work, check this fiddle as an example and this answer as well (about !!), I've used !! in my example, so you may have doubts.

Evaluate prompt value wont

I'm very new to JS - only a couple days in.
Trying to write a very basic prompt evaluated by an if statement.
When I run the code below, the user is prompted, but the statement is never evaluated by the if statement.
Any help? -- I realize the answer is probably simple and obvious, but as a SUPER beginner, what do I do?
var bool = prompt("What is an example of a boolean?");
if (typeof(bool) === "boolean") {
print("correct! that is a boolean");
print(bool) ;
};

In this case, assuming the user inputs something in the prompt, the type of the bool variable will always be a string. You'd rather check if the input compares to the string "true" or "false" etc., like this:
if (bool.toLowerCase() == "true" || bool.toLowerCase() == "false") {
...
}

Why is angular.isNumber() not working as expected?

It appears as if AngularJS's angular.isNumber is not working. It doesn't work with strings that are numbers. Am I doing something wrong? Should I just use isNaN()?
angular.isNumber('95.55') == false
angular.isNumber('95.55' * 1) == true
angular.isNumber('bla' * 1) == true
angular.isNumber(NaN) == true
I need something to see if a string is a number (when it actually is) and angular.isNumber() won't let me do that unless I multiply by 1, but if I do that then it will always be true. Also NaN is not a number (by definition) and so should return false.

In JavaScript, typeof NaN === 'number'.
If you need to recognise a String as a Number, cast it to Number, convert back to String and compare this against the input, for example.
function stringIsNumber(s) {
var x = +s; // made cast obvious for demonstration
return x.toString() === s;
}
stringIsNumber('95.55'); // true
stringIsNumber('foo'); // false
// still have
stringIsNumber('NaN'); // true

I was working on the same problem and I was trying to work around that edge case. So I created a slightly different approach.
FIDDLE
function isStringNumber(str) {
var parsed = parseFloat(str);
var casted = +str;
return parsed === casted && !isNaN(parsed) && !isNaN(casted);
}

Use it as below,
angular.isNumber(eval('99.55'))
for other expressions also we may use eval(input).
Note: eval() is a javascript method
Edit:
It is not recommended to use eval(), as document says Never use eval()!
Thanks #Diogo Kollross

Check for bool in JavaScript

I've got the following jQuery (I've got it wrapped in the document ready function and all that, so please know I'm just showing you the inside of the function.
..
var itemIsSold = $("#itemIsSold").val();
alert(itemIsSold);
if(!itemIsSold) {
...
}
where itemIsSold is a hidden input field. I get the value False upper case F when it hits the alert but never enters my next if statement. I know this has to be something stupid simple.

If the input's value contains the string "False", that will not translate into a false boolean value. You will need to actually check for itemIsSold == "False".

Since the value of the hidden input field is a string, !"False" will be evaluated to false. Note that any string other than a string with the length of 0 is treated as true. So you should rather compare the string value to another string value like "False":
if (itemIsSold == "False") {
// …
}