Concept

Using if/else statement is always related to boolean, whether it is true or false. This checking may be called a truthiness check. In Javascript if/else statement not always about boolean data type but also behavior. Variables might have the same behavior astrue or false but not a boolean data type.

In if/else statement, we may state our code like this one:

// condition checking in if statement
if (condition){
    // condition is true
}

// condition checking in if/else statement
if (condition){
    // condition is true
} else{
    // condition is not true
}

In if/else statement, the condition may check whether it is identical or equal or just acting like.

Identical means that it has the same value/behavior and the same data type. If those requirements are fulfilled then it returns to be true. On the other side, equal only means it has the same value/behavior as a true or false. So the if/else statement may act like below:

if (condition == true){
    // condition is true
}

if (condition === true){
    // condition is a boolean with a true value
}else {
    // condition is not boolean or does not have the true value
}

Different with acting like, we will learn in case.

Cheatsheet

Now let's check code below:

// Below will have the same behavior as false
var isUndefined = undefined;
var isNull = null;
var isZero = 0;
var isZeroFloat = 0.0;
var isEmptyString = "";

All those variables will have the same behavior as false but not identical or equal to false. This is a little bit strange and confusing, so let's take an example. Now we check the undefined variable.

// undefined have the same behavior as false
if (!isUndefined){
    console.log('isUndefined act as false'); // This log will show
}

// undefined is not equal to false
if (isUndefined == false){
    console.log('isUndefined is not equal to false'); // This log will show
}

// but undefined is not identical to a boolean in a strict comparison even it 
// have same value/behavior as false
if (isUndefined === false){
    console.log('isUndefined is not same with false'); // this log won't show
}

See, in the first if condition, undefined act like false but if we check if undefined is equal to false, the result is not. To better understand, I resume those variable results in the table below.

Look how different data types have different conditional results in Javascript.

If you curious about javascript strange behavior, you could visit JavascriptWTF.