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

I think you have basic JavaScript methodology question here.

Now, JavaScript is a loosely typed language. As such, the way and manner in which it treats logical operations differs from that of other standard languages like Java and C++. JavaScript uses a concept known as "type coercion" to determine the value of a logical operation and always returns the value of the first true type. For instance, take a look at the code below:

var x = mystuff || document;
// after execution of the line above, x = document

This is because mystuff is an a priori undefined entity which will always evaluate to false when tested and as such, JavaScript skips this and tests the next entity for a true value. Since the document object is known to JavaScript, it returns a true value and JavaScript returns this object.

If you wanted a boolean value returned to you, you would have to pass your logical condition statement to a function like so:

var condition1 = mystuff || document;

function returnBool(cond){
  if(typeof(cond) != 'boolean'){ //the condition type will return 'object' in this case
     return new Boolean(cond).valueOf();
  }else{ return; }
}    
// Then we test...
var condition2 = returnBool(condition1);
window.console.log(typeof(condition2)); // outputs 'boolean'