Why don't logical operators (&& and ||) always return a boolean result?

Question

Why do these logical operators return an object and not a boolean?

var _ = (obj.fn && obj.fn() ) || obj._ || ( obj._ = {} );

var _ = obj &&          


        

Answer 1

You should think of the short-circuit operators as conditionals rather than logical operators.

x || y roughly corresponds to:

if ( x ) { return x; } else { return y; }  

and x && y roughly corresponds to:

if ( x ) { return y; } else { return x; }  

Given this, the result is perfectly understandable.

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From MDN documentation:

Logical operators are typically used with Boolean (logical) values. When they are, they return a Boolean value. However, the && and || operators actually return the value of one of the specified operands, so if these operators are used with non-Boolean values, they will return a non-Boolean value.

And here's the table with the returned values of all logical operators.