Ruby's ||= (Or Equals) in JavaScript

Ruby's ||= (or equals) in JavaScript?

Both are absolutely correct, but if you are looking for something that works like ||= in ruby. The first method which is variable = variable || {} is the one you are looking for :)

Does Javascript have a conditional assignment like Ruby's ||= (Double pipe)

var x = x || newValue;

This mean whenever x has something being evaluated as false (undefined, null, 0), it will assign the second value.

What does ||= (or-equals) mean in Ruby?

This question has been discussed so often on the Ruby mailing-lists and Ruby blogs that there are now even threads on the Ruby mailing-list whose only purpose is to collect links to all the other threads on the Ruby mailing-list that discuss this issue.

Here's one: The definitive list of ||= (OR Equal) threads and pages

If you really want to know what is going on, take a look at Section 11.4.2.3 "Abbreviated assignments" of the Ruby Language Draft Specification.

As a first approximation,

a ||= b

is equivalent to

a || a = b

and not equivalent to

a = a || b

However, that is only a first approximation, especially if a is undefined. The semantics also differ depending on whether it is a simple variable assignment, a method assignment or an indexing assignment:

a    ||= b
a.c  ||= b
a[c] ||= b

are all treated differently.

Bash equivalent of Ruby and JavaScript OR when initializing variables

In bash you can use the Assign Default Values parameter expansion operator.

: ${a:=1.0}

${parameter:=word}
If parameter is unset or null, the expansion of word is assigned to parameter.

The : command is a no-op, it's used just so we can perform the parameter expansion in its argument list.

Is there an && Equals operator in Ruby

If you are using ruby > 2.3.0 you can use the &. operator: x&.== y. It basically does what .try does in Rails if the value of the operand is different than nil it calls the method and returns its result. If the value is nil it returns nil so you can do things like: do_i_exist&.does_the_result_exist&.nil?

See: What does &. (ampersand dot) mean in Ruby?

Ruby's variable equals if variable equals method?

Yes, it can be confusing, in part because one's first reaction to seeing if identifier = chunk[IDENTIFIER, 1] may be that it's probably a bug, that the author meant if identifier == chunk[IDENTIFIER, 1].

The code block given in the question is probably equivalent to:

tokenizer =
  if chunk[IDENTIFIER, 1]
    IdentifierTokenizer.new(chunk[IDENTIFIER, 1], tokenizer).tokenize
  elsif chunk[CONSTANT, 1]
    ConstantTokenizer.new(chunk[CONSTANT, 1], tokenizer).tokenize
  elsif chunk[NUMBER, 1]
    NumberTokenizer.new(chunk[NUMBER, 1], tokenizer).tokenize
  ..

Rather than computing chunk[IDENTIFIER, 1], chunk[CONSTANT, 1] and chunk[NUMBER, 1] twice, those values are assigned to variables (identifier, constant, number) the first time they are calculated. (Those assignments are evaluated before if and elsif are applied.) This is commonly done when such calculations are costly or have an undesired side effect when performed more than once. The use of temporary variables in this way also DRYs the code, but other approaches that achieve the same objective may be preferable. I personally avoid such inline variable assignments, in part because I regard them as ugly.

One alternative that may be more clear, and which DRYs the code even more, is the following.

tokenizer =
  tokenit(IDENTIFIER, IdentifierTokenize, tokenizer) ||
  tokenit(CONSTANT,   ConstantTokenize,   tokenizer) ||
  tokenit(NUMBER,     NumberTokenize,     tokenizer) ||
  ...

def tokenit(type, class, tokenizer)
  ch = chunk[type, 1]
  if ch
    class.public_send(:new, ch, tokenizer).tokenize
  else
    false
  end
end

This of course assumes that when ch is truthy class.public_send(:new, ch, tokenizer).tokenize is also truthy.

Double Pipe Symbols in Ruby Variable Assignment?

It's a conditional assignment. From here:

 x = find_something() #=>nil
 x ||= "default"      #=>"default" : value of x will be replaced with "default", but only if x is nil or false
 x ||= "other"        #=>"default" : value of x is not replaced if it already is other than nil or false

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