Checking if a variable is defined?

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How can I check whether a variable is defined in Ruby? Is there an isset-type method available?

Use the defined? keyword (documentation). It will return a String with the kind of the item, or nil if it doesn’t exist.

>> a = 1
 => 1
>> defined? a
 => "local-variable"
>> defined? b
 => nil
>> defined? nil
 => "nil"
>> defined? String
 => "constant"
>> defined? 1
 => "expression"

As skalee commented: "It is worth noting that variable which is set to nil is initialized."

>> n = nil  
>> defined? n
 => "local-variable"

How to Check If a Variable Exists or Defined in JavaScript, Answer: Use the typeof operator​​ If you want to check whether a variable has been initialized or defined (i.e. test whether a variable has been declared and assigned a value) you can use the typeof operator. When working with variables what you want most of the time is to check for nil, instead of whether or not a variable is defined. Or even better, convert the variable into a known class using a conversion method.

This is useful if you want to do nothing if it does exist but create it if it doesn't exist.

def get_var
  @var ||=

This only creates the new instance once. After that it just keeps returning the var.

How to Determine If Variable is Undefined or NULL in JavaScript, How do you check if a variable is defined in JS? It is considered unusual in Python not to know whether a variable has already been defined. But if you are nevertheless in this situation, you can make sure that a given variable is in fact defined (as None, if nothing else) by attempting to access it inside a try clause and assigning it the None object if

The correct syntax for the above statement is:

if (defined?(var)).nil? # will now return true or false
 print "var is not defined\n".color(:red)
 print "var is defined\n".color(:green)

substituting (var) with your variable. This syntax will return a true/false value for evaluation in the if statement.

Environment Variables, How do you check if a variable is defined or not in shell script? There is no difference between environment variables and variables in a script. Environment variables are just defined earlier, outside the script, before the script is called. From the script's point of view, a variable is a variable.

defined?(your_var) will work. Depending on what you're doing you can also do something like your_var.nil?

Check if variable is exists in JavaScript –, How do I check if an environment variable is set? If the parameter is not set, you get a check of x==x. If the parameter is set (to, say, “asdf”), you get a check of asdfx==x

Try "unless" instead of "if"

a = "apple"
# Note that b is not declared
c = nil

unless defined? a
    puts "a is not defined"

unless defined? b
    puts "b is not defined"

unless defined? c
    puts "c is not defined"

Testing if a Variable Is Defined - Python Cookbook [Book], To check if a variable is undefined or null you can use the equality operator == or strict equality operator === (also called identity operator). Let's take a look at  {% if variable is defined %} is true if the variable is None. is really your only option. In the Environment setup, we had undefined = StrictUndefined, which prevented undefined values from being set to anything.

Testing if a variable is defined « Python recipes « ActiveState Code, So, in this case, to check if a variable is exists or defined, use “typeof” operator and check if the returned value is “undefined“. 1. typeof == “  exist name returns the type of name as a number. This list describes the type associated with each value: 0 — name does not exist or cannot be found for other reasons. For example, if name exists in a restricted folder to which MATLAB ® does not have access, exist returns 0. 1 — name is a variable in the workspace.

How to Check If a Variable is Defined in Ruby, In Python, all variables are expected to be defined before use. The None object is a value you often assign to signify that you have no real value for a variable,  It depends on the definition of "variable is set". If you want to check if the variable is set to a non-zero string, then mbranning answer is correct. But if you want to check if the variable is declared but not initialized (i.e foo=), then Russel's answer is correct. – Flow Oct 13 '13 at 15:38

How to check if a JavaScript object property is undefined, Python doesn't have a specific function to test whether a variable is defined, since all variables are expected to have been defined before use  However, any variable that you create will automatically become a property of the window object. Therefor, checking if the variable has been added to the scope of window will result in a condition that can be checked. Also note that this will only work in a browser, which of course is how most people experience javascript anyway.