What is the meaning of || in javascript?

Question

I am looking at these lines of code from here:

    if (callback)
        callback(sig || graph);

I have never see vertical "or" bars in a javascript method call. What do they mean? Do they pass the "true" parameter (i.e. sig or graph)? Do they pass the defined parameter? I have never seen that syntax before.

Answer 1

This is the logical OR operator in JS (and most other languages). It is defined in the spec at 11.11. As noted in the spec, expressions on either side will be evaluated first and the logical OR is left-to-right associative. Note that evaluation of the operands follows standard ToBoolean semantics from section 9.2, so [null, undefined, 0, ''] all count as falsy.

Unlike most languages, JS returns the left operand if it is truthy or the right operand otherwise. This behavior has been covered before in a number of SO questions, but is worth noting as most languages simply return true or false. This behavior is often used to provide default values to otherwise undefined variables.

Answer 2

The Logical OR operator (||) is an operator that returns its first or second operand depending on whether the first is truthy. A "truthy" value means anything other than 0, undefined, null, "", or false.

This operator uses short-circuiting, meaning if the first expression is truthy, then the second expression is not evaluated and the first operand is returned immediately. This is akin to the Logical AND operator (&&), which does the opposite: if the first operand is falsey, it returns it, otherwise it returns the second expression.