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How do I compare code to one another in a console?

I am new to learning code, so I do not know what to do. Through my course online I discovered that you can compare text using ==
So when I put "yes" == "yes" the console will say True
It's a pretty neat feature, but back on topic.
How can I compare code to one another? For example, I want to compare:
"The file located at \"C:\\Desktop\My Documents\Roster\names.txt\" contains the names on the roster."
To another code, but with it telling me if it's similar or not. How do I do it? I read somewhere about using Escaping Strings, but I don't know where to place them.
I appreciate the help,
Zeke

Testing for equality compares data on each side of the operator.
== is the loose equality operator.
=== is the strict equality operator.
They differ in how they respond to arguments.
Example:
The string "9" and the number 9 are evaluated by loose equality operator as true
"9" == 9 // returns true
However, the strict equality operator, as its name might suggest, also compares the type of data (string vs. number)
"9" === 9 // returns false
More examples and cases can be found here https://developer.mozilla.org/en-US/docs/Web/JavaScript/Equality_comparisons_and_sameness
For comparing the contents of a file, my suggestion in your case would be to parse the file from .json, so that you can treat it as a JavaScript object. Then you can compare the contents with strings of names.

! operator in Javascript - use cases

I'm just starting to learn Javascript, and am using this reference: https://www.discovermeteor.com/blog/javascript-for-meteor/
There's an interesting commentary that I can't get my head around. I quote below.
An interesting consequence of the ! operator is that it always returns a boolean value, even if what comes after is not a boolean:
a = 12;
!a; // false
This means that if you want to convert a variable to boolean you can just use the ! operator twice (once to force the variable to boolean, a second time to revert the value back):
a = 12;
!!a; // true
Or:
a = 0;
!!a; // false
Can anyone help me makes sense of the wording?
Is it simply trying to say that any integer other than 0 gets assigned a Boolean value of True, and that you can return a Boolean value of True/False by using "!" and "!!" respectively?

Yes, !!something is a way to convert something into a boolean (essentially finding out whether something is truthy). 0 and NaN are the only number values that would convert to false with the !! operator (or Boolean() call). All other numbers will be true.
Similarly, +something is a way to convert something into a number.
I usually prefer the more explicit approach: Boolean(something) and Number(something).

What does !!~ do? [duplicate]

This question already exists:
What does `!!~` mean in javascript? [duplicate]
Closed 8 years ago.
I was reading this blog post which mentioned using:
!!~
I have no idea what this does? at first I thought it would give an error, but the code below does run:
var _sessions = [
"_SID_1",
"_SID_2",
"_SID_3",
"_SID_4"
];
if(!!~_sessions.indexOf("_SID_5")) {
console.log('found');
} else {
console.log('!found');
}
output:
node test.js
!found

~ is the bitwise not operator. It inverts the bits of its operand. ! is the logical not operator. The bitwise not operator will return 0 when applied to -1, which is what indexOf returns when the value is not found in the array. Since 0 evaluates to false, doubly negating it will simply return false (a boolean value, rather than a numeric one):
var index = _sessions.indexOf("_SID_5");
console.log(~index); // 0
console.log(!~index); // true
console.log(!!~index); //false
The bitwise not operator will return a value less than 0 for any other possible value returned by indexOf. Since any other value will evaluate to true, it's just a shorthand method (kind of... they are both the same number of characters!) of checking whether an element exists in an array, rather than explicitly comparing with -1:
if (_sessions.indexOf("_SID_5") > -1) {
// This would work the same way
}
Update
With regards to the performance of this, it appears (in Chrome at least) to be marginally slower than the more common comparison with -1 (which itself is marginally slower than a comparison with 0).
Here's a test case and here's the results:
Update 2
In fact, the code in your question can be shortened, which may have been what the author was attempting to do. You can simply remove the !!, since the ~ will always result in 0 or below (and 0 is the only value that will evaluate to false):
if (~_sessions.indexOf("_SID_5")) {
// This works too
}
However, in a slightly different situation it could make sense to add in the ! operators. If you were to store the result of the bitwise operator in a variable, it would be a numeric value. By applying the logical not operator, you get a boolean value (and applying it again ensures you get the correct boolean value). If for some reason you require a boolean value over a numeric one, it makes a little bit more sense (but you can still just use the normal comparison with -1 or 0):
var inArray = !!~_sessions.indexOf("_SID_5");
console.log(typeof inArray); // boolean

Donald Knuth: "[...] premature optimization is the root of all evil"
For the sake of readability: please use
.indexOf !== -1

This explains it well:
The tilde operator in Javascript
Mixing the two NOT operators together can produce some interesting results:
!~(-2) = false
!~(-1) = true
!~(0) = false
!~(1) = false
!~(2) = false
So this just checks if the value equals -1 or not, and indexOf returns -1 if it does not find a match

What is the best JS practice to check undefined variable? [duplicate]

This question already has answers here:
Closed 11 years ago.
Possible Duplicates:
Javascript: undefined !== undefined?
What is the best way to compare a value against 'undefined'?
I've played around with the console and got some strange results when checking undefined,
when I do var a; a's type and value become "undefined" right?
So why a===undefined is true and a=="undefined" or a==="undefined" are false?
and, would typeof a == "undefined" be the best practice like in other languages?
Unrelated - how do I markup code in a question from iPhone?

When doing a=="undefined" or a==="undefined" you're comparing the value of a with a string which contains the characters u, n, d, e, f, i, n, e, d.
So your expression boils down to undefined=="somestring", which is obviously false.
typeof returns a string, so in this case comparing it to a string works.

I suppose the best way is to perform strict equation check like a === undefined while typeof a == 'undefined' is overkill since there are no (at least as I know) situation which can lead to evaluating a === undefined to false while a is actually have a value of undefined.
I think comparsion of strings and taking typeof from variable is much slower than a strict equation (possibly speed tests needed).
Considering situation expression a itself is suitable way to check a for undefined value except for cases in which you need to handle false value of variable.

=== means compare type and value in Javascript. So
0 == '0' // true, because it is essentially toStringing both values
0 === '0' // false, because one is a Number and one is a String
When you check for a == "undefined" You are seeing if a is equal to the String value "undefined". undefined without quotes in Javascript is an undefined value. a === undefined compares a to the value undefined, and a === "undefined" compares a to the string "undefined".
Using a === undefined is a good practice for checking for definition
edit: this answer has some flaws, which I leave to the commenters to correct me

Just to cover one point: The word "undefined" is not special in javascript. There is no keyword or global representing it.
So when you do a === undefined it returns true because neither name has any value assigned to it - if you had somewhere previously created and assigned a variable with that name (like undefined = 1) then that statement would be false.

Can someone explain this 'double negative' trick? [duplicate]

This question already has answers here:
What is the !! (not not) operator in JavaScript?
(42 answers)
Closed 8 years ago.
I am by no means an expert at Javascript, but I have been reading Mark Pilgrim's "Dive into HTML5" webpage and he mentioned something that I would like a better understanding of.
He states:
Finally, you use the double-negative trick to force the result to a Boolean value (true or false).
function supports_canvas() {
return !!document.createElement('canvas').getContext;
}
If anyone can explain this a little better I would appreciate it!

A logical NOT operator ! converts a value to a boolean that is the opposite of its logical value.
The second ! converts the previous boolean result back to the boolean representation of its original logical value.
From these docs for the Logical NOT operator:
Returns false if its single operand can be converted to true; otherwise, returns true.
So if getContext gives you a "falsey" value, the !! will make it return the boolean value false. Otherwise it will return true.
The "falsey" values are:
false
NaN
undefined
null
"" (empty string)
0

Javascript has a confusing set of rules for what is considered "true" and "false" when placed in a context where a Boolean is expected. But the logical-NOT operator, !, always produces a proper Boolean value (one of the constants true and false). By chaining two of them, the idiom !!expression produces a proper Boolean with the same truthiness as the original expression.
Why would you bother? Because it makes functions like the one you show more predictable. If it didn't have the double negative in there, it might return undefined, a Function object, or something not entirely unlike a Function object. If the caller of this function does something weird with the return value, the overall code might misbehave ("weird" here means "anything but an operation that enforces Boolean context"). The double-negative idiom prevents this.

In javascript, using the "bang" operator (!) will return true if the given value is true, 1, not null, etc. It will return false if the value is undefined, null, 0, or an empty string.
So the bang operator will always return a boolean value, but it will represent the opposite value of what you began with. If you take the result of that operation and "bang" it again, you can reverse it again, but still end up with a boolean (and not undefined, null, etc).
Using the bang twice will take a value that could have been undefined, null, etc, and make it just plain false. It will take a value that could have been 1, "true", etc. and make it just plain true.
The code could have been written:
var context = document.createElement('canvas').getContext;
var contextDoesNotExist = !context;
var contextExists = !contextDoesNotExist;
return contextExists;

Using !!variable gives you a guarantee of typecast to boolean.
To give you a simple example:
"" == false (is true)
"" === false (is false)
!!"" == false (is true)
!!"" === false (is true)
But it doesn't make sense to use if you are doing something like:
var a = ""; // or a = null; or a = undefined ...
if(!!a){
...
The if will cast it to boolean so there is no need to make the implicit double negative cast.

! casts "something"/"anything" to a boolean.
!! gives the original boolean value back (and guarantees the expression is a boolean now, regardless to what is was before)

The first ! coerces the variable to a boolean type and inverts it. The second ! inverts it again (giving you the original (correct) boolean value for whatever you are checking).
For clarity you would be better off using
return Boolean(....);

document.createElement('canvas').getContext may evaluate to either undefined or an object reference. !undefined yields true, ![some_object] yields false. This is almost what we need, just inverted. So !! serves to convert undefined to false and an object reference to true.

It's to do with JavaScript's weak typing. document.createElement('canvas').getContext is a function object. By prepending a single ! it evaluates it as a boolean expression and flips the answer around. By prepending another !, it flips the answer back. The end result is that the function evaluates it as a boolean expression, but returns an actual boolean result rather than the function object itself. Prepending !! is a quick and dirty way to typecast an expression to a boolean type.

If document.createElement('canvas').getContext isn't undefined or null, it will return true. Otherwise it will return false.