How Can JavaScript Logical Operators Return Objects?

Understanding the Logical Operators in JavaScript: Why They Can Return Objects

In JavaScript, logical operators like && (and) and || (or) are typically expected to evaluate to boolean values (true or false). However, in certain scenarios, they can also return objects. This surprising behavior can initially leave you perplexed.

To delve into this phenomenon, let's analyze the following code snippets:

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

var _ = obj && obj._;

In the first example, obj.fn is initially evaluated. If it's defined, the result of the expression obj.fn() is returned. Otherwise, the expression obj._ is evaluated, or a new object is created if neither obj.fn nor obj._ exist.

Similarly, in the second example, obj is evaluated first. If obj exists (evaluates to true), the result is the value of obj._. However, if obj doesn't exist (evaluates to false), the original obj value itself is returned.

Unveiling the Logical Short-Circuit Behavior

This behavior stems from the logical short-circuit nature of && and ||. In other words, these operators attempt to return a "logical value" as quickly as possible, avoiding unnecessary evaluations where the result is already clear.

In the case of ||, if the left-hand expression evaluates to true, it's instantly returned, and the right-hand expression is not evaluated at all. Conversely, if the left-hand expression evaluates to false, the right-hand expression is evaluated and returned.

Falsehood and Value Return in JavaScript

The other aspect that influences this behavior is how falsy values are treated in JavaScript. Falsy values in JavaScript include false, 0, NaN, empty strings, null, and undefined.

Prior to version 1.2 of JavaScript, || returned false when the expression evaluated to false. However, from version 1.2 onwards, it returns the actual value of the expression. This means that if the left-hand expression is false, the value of the right-hand expression will be returned.

Similarly, for &&, if the left-hand expression evaluates to true, the right-hand expression is evaluated, and its value is returned. However, if the left-hand expression evaluates to false, false is returned in JS 1.0 and 1.1, while the value of the left-hand expression is returned from JS 1.2 onwards.

Conclusion

Understanding the logical short-circuit behavior and JavaScript's treatment of falsy values is crucial for comprehending the sometimes-surprising return values of logical operators. By recognizing that these operators prioritize efficiency and that falsehood can have different interpretations in JavaScript, you can navigate the complexities of these operators with confidence.

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