Why Do JavaScript Logical Operators Sometimes Return Objects Instead of Booleans?

Logical Operators in JavaScript: Understanding Non-Boolean Results

Unlike mathematical operators, which always return numerical values, logical operators (&& and ||) in JavaScript can sometimes return objects instead of booleans. This can be counterintuitive, especially when trying to use them for conditional checks.

Why Objects are Returned

The || and && operators are logical short-circuit operators, meaning they evaluate their operands from left to right and return the first fully-determined logical value. In JavaScript, this value can be either true or false, but it can also be an object.

Understanding the Conditional Logic

In the expression X || Y, X is evaluated first and interpreted as a boolean value. If X is true, it is returned immediately, and Y is not evaluated. This is because the expression has already been determined to be true.

If X is false, Y is then evaluated and its boolean value is returned.

Similarly, && stops evaluating if the first argument is false.

The Trick with "False"

The first point of confusion arises because when an expression is evaluated as "true," the expression itself is returned. This is why you might see actual values being returned from logical expressions.

The Trick with Null

The second point of confusion stems from JavaScript's behavior in different versions. Initially, it would return false for "false" expressions, but from version 1.2 onward, it returns the actual value of the expression.

Examples

To illustrate:

var _ = (obj.fn && obj.fn()) || obj._ || (obj._ = {}); // Returns obj.fn() if defined, otherwise obj._
var _ = obj && obj._; // Returns obj._ if obj is true, otherwise obj

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