How to check for an undefined or null variable in JavaScript?

You have to differentiate between cases:

1. Variables can be undefined or undeclared. You'll get an error if you access an undeclared variable in any context other than typeof.

if(typeof someUndeclaredVar == whatever) // works
if(someUndeclaredVar) // throws error

A variable that has been declared but not initialized is undefined.

let foo;
if (foo) //evaluates to false because foo === undefined

2. Undefined properties , like someExistingObj.someUndefProperty. An undefined property doesn't yield an error and simply returns undefined, which, when converted to a boolean, evaluates to false. So, if you don't care about 0 and false, using if(obj.undefProp) is ok. There's a common idiom based on this fact:

   value = obj.prop || defaultValue
   

   which means "if obj has the property prop, assign it to value, otherwise assign the default value defautValue".

   Some people consider this behavior confusing, arguing that it leads to hard-to-find errors and recommend using the in operator instead

   value = ('prop' in obj) ? obj.prop : defaultValue
   

I think the most efficient way to 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 ) {}

Note 1

As mentioned in the question, the short variant requires that some_variable has been declared, otherwise a ReferenceError will be thrown. However in many use cases you can assume that this is safe:

check for optional arguments:

function(foo){
    if( foo == null ) {...}

check for properties on an existing object

if(my_obj.foo == null) {...}

On the other hand typeof can deal with undeclared global variables (simply returns undefined). Yet these cases should be reduced to a minimum for good reasons, as Alsciende explained.

Note 2

This - even shorter - variant is not equivalent:

if ( !some_variable ) {
  // some_variable is either null, undefined, 0, NaN, false, or an empty string
}

so

if ( some_variable ) {
  // we don't get here if some_variable is null, undefined, 0, NaN, false, or ""
}

Note 3

In general it is recommended to use === instead of ==. The proposed solution is an exception to this rule. The JSHint syntax checker even provides the eqnull option for this reason.

From the jQuery style guide:

Strict equality checks (===) should be used in favor of ==. The only exception is when checking for undefined and null by way of null.

// Check for both undefined and null values, for some important reason. 
undefOrNull == null;

EDIT 2021-03:

Nowadays most browsers support the Nullish coalescing operator (??) and the Logical nullish assignment (??=), which allows a more concise way to assign a default value if a variable is null or undefined, for example:

if (a.speed == null) {
  // Set default if null or undefined
  a.speed = 42;
}

can be written as any of these forms

a.speed ??= 42;
a.speed ?? a.speed = 42;
a.speed = a.speed ?? 42;