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javascriptparametersoptional-parametersor-operator

What does the construct x = x || y mean?

I am debugging some JavaScript and can't explain what this || does:

function (title, msg) {
  var title = title || 'Error';
  var msg   = msg || 'Error on Request';
}

Why is this guy using var title = title || 'ERROR'? I sometimes see it without a var declaration as well.

Solution

  • It means the title argument is optional. So if you call the method with no arguments it will use a default value of "Error".

    It's shorthand for writing:

    if (!title) {
  title = "Error";
}

    This kind of shorthand trick with boolean expressions is common in Perl too. With the expression:

    a OR b

    it evaluates to true if either a or b is true. So if a is true you don't need to check b at all. This is called short-circuit boolean evaluation so:

    var title = title || "Error";

    basically checks if title evaluates to false. If it does, it "returns" "Error", otherwise it returns title.