How Can I Reliably Check for Null or Undefined Variables in JavaScript?

Exploring Reliable Null and Undefined Variable Detection in JavaScript

In JavaScript, ascertaining whether a variable is defined or has a value can be a recurring task. Many developers resort to the following pattern:

<code class="javascript">if (typeof(some_variable) != 'undefined' && some_variable != null) {
    // Do something with some_variable
}</code>

While this method is reliable, it can be verbose. Some sources suggest that simply checking if the variable exists has the same effect:

<code class="javascript">if (some_variable) {
    // Do something with some_variable
}</code>

However, certain development environments, such as Firebug, report an error when some_variable is undefined with the second approach.

A more efficient way to check for null or undefined variables is to utilize the following syntax:

<code class="javascript">if (some_variable == null) {
    // some_variable is either null or undefined
}</code>

This alternative is equivalent to the verbose version and is also supported by development tools like Firebug.

Notes:

1. The variable must be declared before using this shorthand technique; otherwise, a ReferenceError will occur. However, it can be safely used for optional arguments or object property checks.
2. A different but not equivalent variant of this check includes variables that are 0, NaN, false, or empty strings:

<code class="javascript">if (!some_variable) {
    // some_variable is either null, undefined, 0, NaN, false, or an empty string
}</code>

3. Using strict equality checks (===) is generally preferred over ==. However, the recommended == null comparison is an exception to this rule.

Update 2021-03:

Modern browsers support the Nullish coalescing operator (??) and Logical nullish assignment (??=), providing a concise way to assign default values if variables are null or undefined:

<code class="javascript">if (a.speed == null) {
    // Set default if null or undefined
    a.speed = 42;
}</code>

This can be rewritten using the Nullish coalescing operator:

<code class="javascript">a.speed ??= 42;</code>

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