Check for null, undefined, or blank variables in JavaScript

value-is-null-or-undefined.js

/**
  Test for "value is null or undefined" is
**/
if ( some_variable == null ){
  // some_variable is either null or undefined
}

// So these two lines are equivalent:
if ( typeof(some_variable) !== "undefined" && some_variable !== null ) {}
if ( some_variable != null ) {}

value-is-truthy.js

/** 
  Test for truthy value, https://stackoverflow.com/a/5515349
**/

// You can just check if the variable has a truthy value or not. That means

if( some_variable ) {
  // some_variable is some_variable
}

// will evaluate to true if value is not:
//    null
//    undefined
//    NaN
//    empty string ("")
//    0
//    false
// The above list represents all possible falsy values in ECMA-/Javascript. Find it in the specification at the ToBoolean section, http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-262.pdf