What Do You Get Back When You Use Logical Operators in JavaScript?

Navigating the Non-Boolean Returns of Logical Operators in JavaScript

While the question focuses on the absence of boolean returns in specific logical expressions, it highlights a broader concept in JavaScript regarding the behavior of the logical operators || (OR) and && (AND).

In JavaScript, || and && are short-circuit operators that terminate early once they encounter a fully determined logical value. This means that the evaluation of the second operand is skipped if the first operand is sufficient to determine the outcome.

For example, in the expression X || Y, if X evaluates to a truthy value (anything that is not explicitly falsey: true, an object, a string, etc.), then X is returned immediately, rendering the evaluation of Y unnecessary.

Similarly, in X && Y, if X evaluates to falsey, then the evaluation stops and X is returned, again bypassing Y.

The quirk arises when the expression evaluates to a falsey value. Prior to JavaScript 1.2, the operator would return the boolean value false. However, from JavaScript 1.2 onwards, the actual evaluated value is returned.

Therefore, in the provided expressions:

1. (obj.fn && obj.fn()) || obj._ || (obj._ = {}): If obj.fn exists and evaluates to a truthy value, the expression will return the result of obj.fn(). Otherwise, it will return obj._ if it's defined or create a new object and assign it to obj._ if obj._ is undefined.
2. obj && obj._: This simply results in obj._ if obj is truthy, otherwise obj is returned.

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