Can the Logical OR (||) Operator in JavaScript Handle Non-Boolean Operands?

Logical OR (||) Operator and Non-Boolean Operands

In JavaScript, the logical OR operator (||) is commonly used to evaluate boolean expressions. However, it can also be applied to non-boolean operands, leading to unexpected results.

Consider the following code:

<code class="javascript">var $time = Date.now || function() {
  return +new Date;
};</code>

In this example, the || operator is used to assign a function to the $time variable. The expression evaluates to the result of Date.now if it exists (i.e., it is not null or undefined), otherwise it executes the anonymous function, which returns the current timestamp.

The reason for this behavior lies in the secondary role of the || operator as a "default" operator. When both operands are non-boolean, it returns the left-hand operand if it is truthy (not null, undefined, the empty string, or 0), and the right-hand operand if the left-hand operand is falsy.

In the given code, Date.now is a truthy value, so the left-hand operand is returned, which is the original Date.now function. However, if Date.now was not available (e.g., in older browsers), the right-hand operand, which is the anonymous function, would have been executed instead.

This usage of the || operator allows developers to specify default values or fallback behaviors in a concise way. For example, the following code assigns a value to the v variable, but if v is not defined, it sets the value to 10:

<code class="javascript">value = v || 10;</code>

Similarly, the following code guards against accessing a property of a nullable object:

<code class="javascript">var value = p &amp;&amp; p.name;</code>

In conclusion, the || (logical OR) operator in JavaScript can also be used with non-boolean operands, where it evaluates to the left-hand operand if it is truthy, or to the right-hand operand otherwise. This behavior can be useful for specifying default values and writing null-safe code.

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