Caution when using the || operator to set defaults

I often see the use of || to set a default value for a variable that might be nil, null or undefined.

Copyx = x || 'default-value'

This pattern should be avoided in all languages.

While using || works as intended when x is null or an actual object, it also sets the default value for other falsy Show archive.org snapshot values, such as false. false is a non-blank value that you never want to override with a default.

To make it worse, languages like JavaScript or Perl have many more falsey values Show archive.org snapshot , including the number 0 or the empty string ('').

Setting defaults correctly: Ruby

Prefer to check for nil? before overriding a value with a default:

Copyx = 'default-value' if x.nil?

Avoid checking if x.blank? because false is considered "blank" in ActiveSupport.

Defaults for hash values

If you're reading from an options hash, you probably want to use fetch Show archive.org snapshot :

Copyx = options.fetch(:x, 'default-value')

fetch returns the default value only if the key :x is missing from the Hash entirely.

Note

There is a difference between a hash missing a key :x, and a hash that has :x assigned to nil. In the last case, the hash still has a key :x, and fetch will return its value nil.

Setting defaults correctly: JavaScript

Plain ES5 (no libraries)

Prefer to check if the variable is null or undefined before overriding its value with a default:

Copyif (x === null || typeof x === 'undefined') {
  x = 'default-value';
}

ES-next

ES6 has the "nullish coalescing" syntax Show archive.org snapshot :

Copyx = x ?? 'default-value'

If you are using Babel, you can transpile this syntax into ES5 using the @babel/plugin-proposal-nullish-coalescing-operator Show archive.org snapshot package.

With Lodash

If you are using Underscore or Lodash Show archive.org snapshot you can also write this as:

Copyif (_.isUndefined(x) || _.isNull(x)) {
  x = 'default-value';
}

If you are using a recent version of Lodash you can also write this as:

Copyif (_.isNil(x)) {
  x = 'default-value';
}

This is also possible in recent Lodash versions (but slower if your default value is expensive to construct):

Copyx = _.defaultTo(x, 'default-value')

With CoffeeScript

CoffeeScript lets you write:

Copyx ?= 'default-value'