Truthy and Falsy

First, we need to understand the truthy and falsy concept, because it will be widely used in the second part.

Truthy and falsy is a common concept among dynamic language featuring loose typing.

Any value, whatever the type, belong to the truthy or falsy group. When an operator need a boolean value, truthy will be true and falsy will be false.

Those values are falsy:

undefined  
null  
false  
0  
NaN  
''  

Those values are truthy:

true  
1  
123    // any numbers that are not 0 are truthy  
'some random string'    // any non-empty string are truthy  
[]    // any array are truthy, even empty arrays
{}    // any object are truthy, even empty objects, except null (yay, null is an object...)

An if statement will execute its block of code if the expression is truthy.

if ( expression )  
{
    // if expression is truthy, the following lines are executed
    ...
}
else  
{
    // if expression is falsy, the following lines are executed
    ...
}

Syntactic Sugar

Here an unordered list of shortcuts that eases life and helps maintaining a neat and elegant codebase.

• !! expression: a very elegant way to cast a variable or an expression into a boolean, much more pleasant than the cumbersome expression ? true : false. It is also known as the double not. How it works? ! returns the inverse boolean value of an expression. Apply it twice and you are back to the original logical value of the expression, casted as a boolean.

• + stringExpression: it converts a string expression into a number, e.g. typeof + '42' === 'number' is true. It works if + is used as an unary operator, when there are no left member.

• + new Date(): it converts the Date value into a number, same mechanics than the previous one.

• optionalArgument || defaultValue: it returns optionalArgument if it is truthy, else it returns defaultValue. || is sometime refered as the default operator. In Javascript, the || and && operator are a bit like flow controle, they don't produce booleans, but instead they return the last evaluated operand. Still, the returned operand is considerated truthy or falsy according to the logical rules of AND and OR operators.

  Also, the first variable was named optionalArgument on purpose. Because it is an elegant way to define default parameter for functions when the type of the parameter is expected to be always truthy (object, array). Do not use when falsy values are expected.

function fn( optionalObject )  
{
  optionalObject = optionalObject || {} ;
  ...
}

• object && object.child && object.child.property: this is the guard operator, allowing us to safely access the property of nested objects. Without that, accessing directly object.child.property would raise an error if object was not an object, or if object has no child property.

• !a !== !b: this is the easiest way to achieve a logical XOR (eXclusive OR) in Javascript. This expression is true if a is truthy or b is truthy, but not both of them. The not operator cast a and b to boolean (while reversing their logical value), then if they are different, one and only one of them is true, therefore the whole expression is true.

Feel free to comment and share your own favorite syntactic sugar with us!