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JavaScript JavaScript Loops, Arrays and Objects Simplify Repetitive Tasks with Loops `do ... while` Loops

Alexander Melo
Alexander Melo
4,297 Points
Posted by Alexander Melo
Alexander Melo
4,297 Points

Do While Loops -- Difference between 'not' operators?

Guil in this video uses the ! correctGuess. I was wondering if this is the same as writing correctGuess !== true ?? Please tell me the difference between these two operators.

4 Answers

Hugo Paz
Hugo Paz
15,622 Points
Hugo Paz
Hugo Paz
15,622 Points

Hi Alexander,

They are similar but there is a crucial difference. The !correctGuess is not strict like the !== (or ===).

Let me give you an example:

var check = 0;
//case 1
if(!check){
console.log('In this case 0 is evaluated to false');
}
//case 2
if(check==false){
console.log('In this case 0 is evaluated to false as well');
}
//case 3
if(check===false){
//this will fail because 0 is not strictly false
}

So if correctGuess is a boolean value, both options are the same. But if you use a value, like 0, both will work differently as you saw above.

Marcus Parsons
Marcus Parsons
15,719 Points
Marcus Parsons
Marcus Parsons
15,719 Points

Hey Alexander,

Using !variableName is a way to check that a variable contains any falsey values. Falsey values are: false, 0 (or 0.0), "", null, NaN, and undefined. It's a good catch all to check to see if a variable is empty, not declared, not false, etc. and then do some code if it is one of those things.

if (!correctGuess) {
//do code
}

//The above is the same as writing:
if (correctGuess === false || correctGuess === "" || correctGuess === 0 || correctGuess === null || correctGuess === NaN || correctGuess === undefined) {
//do code
}

As you can see, writing !correctGuess is a much faster and more efficient way of checking to make sure something is a falsey value than writing out all of those OR statements.

Here are some logic tests that you can copy and paste into your web console (or on a test page):

var a = 0;
if (!a) {
alert(a + ' is a falsey value!');
}

var b = false;
if (!b) {
alert(b + ' is a falsey value!');
}

var c = "";
if (!c) {
alert(c + ' is a falsey value!');
}

var d = null;
if (!d) {
alert(d + ' is a falsey value!');
}

var e = NaN;
if (!e) {
alert(e + ' is a falsey value!');
}

var f; //undefined
if (!f) {
alert(f + ' is a falsey value!');
}
derekverrilli
derekverrilli
8,841 Points
derekverrilli
derekverrilli
8,841 Points

Yes, they both mean the same thing.

An aside, Dave McFarland teaches this course, not Guil Hernandez ;)

derekverrilli
derekverrilli
8,841 Points
derekverrilli
derekverrilli
8,841 Points

Hugo explained it much better.

ellie adam
ellie adam
26,377 Points
ellie adam
ellie adam
26,377 Points

Hi Alexander,

These are basic operators.

== equal to x == 8 false
x == 5 true
=== equal value and equal type x === "5" false
x === 5 true
!= not equal x != 8 true
!== not equal value or not equal type x !== "5" true x !== 5 false

greater than x > 8 false < less than x < 8 true
= greater than or equal to x >= 8 false
<= less than or equal to x <= 8 true

http://www.w3schools.com/js/js_comparisons.asp https://docs.oracle.com/javase/tutorial/java/nutsandbolts/while.html