Is Checking for Null or Undefined in JavaScript Really That Simple?

Determining Value Presence in JavaScript: Undefined versus Null

In JavaScript, variables can store various data types, including null and undefined. Distinguishing between these two values is crucial for maintaining code integrity.

Consider the following code snippet:

if (typeof(some_variable) != 'undefined' && some_variable != null) {
  // Do something with some_variable
}

While this pattern is commonly used, it lacks conciseness. A simpler alternative exists:

if (some_variable) {
  // Do something with some_variable
}

However, Firebug may raise errors when using this simplified form for undefined variables. This behavior prompts the question: Are these two methods truly interchangeable?

Defining Variable Presence

The most efficient way to check for null or undefined values is through the following:

if (some_variable == null) {
  // some_variable is either null or undefined
}

This method equates to the following:

if (typeof(some_variable) !== "undefined" && some_variable !== null) {}
if (some_variable != null) {}

Note that the simplified form assumes variable declaration; otherwise, a ReferenceError will occur. This assumption is often safe in contexts like checking for optional arguments or properties on existing objects.

Alternatively, the following form is not equivalent:

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

Note: It's generally recommended to use === instead of ==, with the proposed solution being an exception.

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