JavaScript conditional assignment

A common idiom with in programming is conditional assignment, usually to set an undefined variable. Ruby has a nice operator, ||= for this purpose, JavaScript doesn’t have such an operator. So often variables that are used across multiple files, like namespaces, have to be repeatedly conditionally assigned. By not doing so you may try to use the variable before it’s been declared and have a ReferenceError thrown.

There are lots of ways to conditionally assign in JavaScript. This is the way I learned from Jeremy Ashkenas, and the way I like to do it:

var Namespace;
Namespace || (Namepace = {});

This can also be used for arguments and local variables,

Namespace.Model = {};

_.extend(Namespace.Model, {
  set: function(attrs, options) {
    options || (options = {});

    if (options.logger) {
      return options.logger.log("Hello. Yes, This is log.")

# => ...Nothing/no ReferenceError!

Namespace.Model.set({}, {logger: console})
# => "Hello. Yes, This is log."

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Written: March 20th, 2012
Categories: javascript, namespaces, "conditional, assignment"