Why Do Logical Operators in JavaScript Sometimes Return Non-Boolean Values?

Understanding the Return Value of Logical Operators

In JavaScript, the logical operators && (logical AND) and || (logical OR) are known for returning boolean results. However, under certain circumstances, they may surprisingly return non-boolean values. This anomaly arises due to the underlying evaluation process of these operators.

Logical Short-Circuit Operators

The key characteristic of && and || lies in their behavior as logical short-circuit operators. When evaluating an expression using these operators, they evaluate their left-hand argument first.

Evaluating || (Logical OR)

In the case of ||, if the left-hand argument evaluates to a fully-determined logical value (true or false), the result is returned without evaluating the right-hand argument. This occurs because the logical outcome has already been determined.

Evaluating && (Logical AND)

Similar to ||, && evaluates the left-hand argument first. However, if the left-hand argument evaluates to false, the expression returns false without evaluating the right-hand argument. This is because && requires both arguments to be true for the expression to be true.

Understanding the Return Value

When a logical expression is evaluated as true, both && and || return the left-hand argument itself. This is because in JavaScript, any expression that evaluates to a non-boolean value is treated as true.

However, if the left-hand argument evaluates to false, the original expression will evaluate to false. In JavaScript version 1.2 and later, the expression itself is returned as the result. This behavior allows for the chaining of logical operations without the need for explicit ternary operators.

Example:

Consider the following code:

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

In this example, if obj.fn is defined and returns a truthy value, the expression obj.fn will be returned. If obj.fn is not defined, the expression will evaluate to false, which will cause the right-hand argument, obj._ || (obj._ = {}), to be evaluated and returned.

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