Reputation: 2229

boolean in an if statement

Today I've gotten a remark about code considering the way I check whether a variable is true or false in a school assignment.

The code which I had written was something like this:

var booleanValue = true;

function someFunction(){
    if(booleanValue === true){
        return "something";
    }
}

They said it was better/neater to write it like this:

var booleanValue = true;

function someFunction(){
    if(booleanValue){
        return "something";
    }
}

The remark which I have gotten about the "=== true" part was that it was not needed and could create confusion.

However my idea is that it is better to check whether the variable is a boolean or not, especially since Javascript is a loosetyped language.

In the second example a string would also return "something";

So my question; Is it neater to loose the "=== true" part in the future, or is it good practise to check the type of the variable as well.

Edit: In my "real" code the boolean represents whether an image has been deleted or not, so the only values boolValue should ever have is true or false.

0 and 1 for example shouldn't be in that variable.

Upvotes: 196

Answers (14)

C0L.PAN1C

Reputation: 12243

The identity (===) operator behaves identically to the equality (==) operator except no type conversion is done, and the types must be the same to be considered equal.

Upvotes: 4

Igor Pavlenko

Reputation: 707

Also can be tested with Boolean object, if you need to test an object error={Boolean(errors.email)}

Upvotes: 2

Alyafey

Reputation: 1453

Since the checked value is Boolean it's preferred to use it directly for less coding and at all it did same ==true

Upvotes: 3

anaid JimMor

Reputation: 61

Revisa https://www.w3schools.com/js/js_comparisons.asp

example:

var p=5;

p==5 ? true
p=="5" ?  true
p==="5" ? false

=== means same type also same value == just same value

Upvotes: 1

jfriend00

Reputation: 707228

First off, the facts:

if (booleanValue)

Will satisfy the if statement for any truthy value of booleanValue including true, any non-zero number, any non-empty string value, any object or array reference, etc...

On the other hand:

if (booleanValue === true)

This will only satisfy the if condition if booleanValue is exactly equal to true. No other truthy value will satisfy it.

On the other hand if you do this:

if (someVar == true)

Then, what Javascript will do is type coerce true to match the type of someVar and then compare the two variables. There are lots of situations where this is likely not what one would intend. Because of this, in most cases you want to avoid == because there's a fairly long set of rules on how Javascript will type coerce two things to be the same type and unless you understand all those rules and can anticipate everything that the JS interpreter might do when given two different types (which most JS developers cannot), you probably want to avoid == entirely.

As an example of how confusing it can be:

var x;

x = 0;
console.log(x == true);   // false, as expected
console.log(x == false);  // true as expected

x = 1;
console.log(x == true);   // true, as expected
console.log(x == false);  // false as expected

x = 2;
console.log(x == true);   // false, ??
console.log(x == false);  // false

For the value 2, you would think that 2 is a truthy value so it would compare favorably to true, but that isn't how the type coercion works. It is converting the right hand value to match the type of the left hand value so its converting true to the number 1 so it's comparing 2 == 1 which is certainly not what you likely intended.

So, buyer beware. It's likely best to avoid == in nearly all cases unless you explicitly know the types you will be comparing and know how all the possible types coercion algorithms work.

So, it really depends upon the expected values for booleanValue and how you want the code to work. If you know in advance that it's only ever going to have a true or false value, then comparing it explicitly with

if (booleanValue === true)

is just extra code and unnecessary and

if (booleanValue)

is more compact and arguably cleaner/better.

If, on the other hand, you don't know what booleanValue might be and you want to test if it is truly set to true with no other automatic type conversions allowed, then

if (booleanValue === true)

is not only a good idea, but required.

For example, if you look at the implementation of .on() in jQuery, it has an optional return value. If the callback returns false, then jQuery will automatically stop propagation of the event. In this specific case, since jQuery wants to ONLY stop propagation if false was returned, they check the return value explicity for === false because they don't want undefined or 0 or "" or anything else that will automatically type-convert to false to also satisfy the comparison.

For example, here's the jQuery event handling callback code:

ret = ( specialHandle || handleObj.handler ).apply( matched.elem, args );

if ( ret !== undefined ) {
     event.result = ret;
     if ( ret === false ) {
         event.preventDefault();
         event.stopPropagation();
     }
 }

You can see that jQuery is explicitly looking for ret === false.

But, there are also many other places in the jQuery code where a simpler check is appropriate given the desire of the code. For example:

// The DOM ready check for Internet Explorer
function doScrollCheck() {
    if ( jQuery.isReady ) {
        return;
    }
    ...

Upvotes: 300

SLaks

Reputation: 887315

In general, it is cleaner and simpler to omit the === true.

However, in Javascript, those statements are different.

if (booleanValue) will execute if booleanValue is truthy – anything other than 0, false, '', NaN, null, and undefined.

if (booleanValue === true) will only execute if booleanValue is precisely equal to true.

Upvotes: 10

karaxuna

Reputation: 26930

If you write: if(x === true) , It will be true for only x = true

If you write: if(x) , it will be true for any x that is not: '' (empty string), false, null, undefined, 0, NaN.

Upvotes: 53

RestingRobot

Reputation: 2978

In Javascript the idea of boolean is fairly ambiguous. Consider this:

 var bool = 0 
 if(bool){..} //evaluates to false

 if(//uninitialized var) //evaluates to false

So when you're using an if statement, (or any other control statement), one does not have to use a "boolean" type var. Therefore, in my opinion, the "=== true" part of your statement is unnecessary if you know it is a boolean, but absolutely necessary if your value is an ambiguous "truthy" var. More on booleans in javscript can be found here.

Upvotes: 2

QuentinUK

Reputation: 3077

In the plain "if" the variable will be coerced to a Boolean and it uses toBoolean on the object:-

    Argument Type   Result

    Undefined       false
    Null            false
    Boolean         The result equals the input argument (no conversion).
    Number          The result is false if the argument is +0, −0, or NaN;
                    otherwise the result is true.
    String          The result is false if the argument is the empty 
                    String (its length is zero); otherwise the result is true.
    Object          true.

But comparison with === does not have any type coercion, so they must be equal without coercion.

If you are saying that the object may not even be a Boolean then you may have to consider more than just true/false.

if(x===true){
...
} else if(x===false){
....
} else {
....
}

Upvotes: 9

Jesse Hallett

Reputation: 2007

I think that your reasoning is sound. But in practice I have found that it is far more common to omit the === comparison. I think that there are three reasons for that:

It does not usually add to the meaning of the expression - that's in cases where the value is known to be boolean anyway.
Because there is a great deal of type-uncertainty in JavaScript, forcing a type check tends to bite you when you get an unexpected undefined or null value. Often you just want your test to fail in such cases. (Though I try to balance this view with the "fail fast" motto).
JavaScript programmers like to play fast-and-loose with types - especially in boolean expressions - because we can.

Consider this example:

var someString = getInput();
var normalized = someString && trim(someString);  
// trim() removes leading and trailing whitespace

if (normalized) {
    submitInput(normalized);
}

I think that this kind of code is not uncommon. It handles cases where getInput() returns undefined, null, or an empty string. Due to the two boolean evaluations submitInput() is only called if the given input is a string that contains non-whitespace characters.

In JavaScript && returns its first argument if it is falsy or its second argument if the first argument is truthy; so normalized will be undefined if someString was undefined and so forth. That means that none of the inputs to the boolean expressions above are actually boolean values.

I know that a lot of programmers who are accustomed to strong type-checking cringe when seeing code like this. But note applying strong typing would likely require explicit checks for null or undefined values, which would clutter up the code. In JavaScript that is not needed.

Upvotes: 2

Barmar

Reputation: 780818

If the variable can only ever take on boolean values, then it's reasonable to use the shorter syntax.

If it can potentially be assigned other types, and you need to distinguish true from 1 or "foo", then you must use === true.

Upvotes: 4

usumoio

Reputation: 3568

This depends. If you are concerned that your variable could end up as something that resolves to TRUE. Then hard checking is a must. Otherwise it is up to you. However, I doubt that the syntax whatever == TRUE would ever confuse anyone who knew what they were doing.

Upvotes: 1

Sunny

Reputation: 4809

Since you already initialized clearly as bool, I think === operator is not required.

Upvotes: 2

Ven

Reputation: 19039

It depends on your usecase. It may make sense to check the type too, but if it's just a flag, it does not.

Upvotes: 5

boolean in an if statement

Answers (14)

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