When is the || Operator Used for Default Setting in JavaScript?

Unveiling the Hidden Usage of the || Operator in Non-Boolean Contexts

In a JavaScript library used for canvas drawing, a peculiar use of the logical OR (||) operator has surfaced. An expression like var $time = Date.now || function() { return new Date; }; sparks a question: is this a variation of the familiar || operator, or is something else at play?

To unravel this mystery, we delve into the depths of Crockford's Survey of the JavaScript Programming Language. There, we discover that the || operator possesses a hidden alias: the default operator. It does not evaluate to a boolean value but instead returns the left or right-hand operand depending on specific conditions.

When the first operand evaluates to false, null, undefined, an empty string, or 0, the default operator steps in and returns the second operand. This unique behavior turns out to be a convenient tool for null-checking and setting default values.

For example, in the $time expression, if Date.now doesn't exist or returns a falsy value, the fallback function is executed to ensure the existence of a time function. Similarly, value = v || 10 assigns the value of v if it's not falsy; otherwise, it uses the default value of 10.

Understanding this alternate role of the || operator clarifies its usage in non-boolean contexts. It's a placeholder, a default setter, that guarantees the availability of a fallback value or feature when necessary.

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