Checking for Undefined or Null Variables in JavaScript

In JavaScript, it's common to encounter variables without an explicitly assigned value or variables that might be unset. This makes checking for undefined or null variables crucial for maintaining code integrity.

Conditional Checking

The traditional approach to check for undefined or null variables involves a conditional statement using the typeof operator and strict equality checks:

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

While verbose, this technique ensures precision by explicitly checking for both undefined and null values. However, some developers prefer a shorthand notation:

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

Simplified Conditional

This simplified notation relies on JavaScript's implicit conversion rules. Any non-falsey value, including defined variables, evaluates to true. Therefore, if some_variable is defined and not null, the condition will be true.

However, this shorthand can lead to unexpected behavior in certain situations. For example, Firebug may display an error when some_variable is undefined, whereas the more verbose conditional will execute without issue.

Recommended Approach

The most reliable way to check for undefined or null values is to use the strict equality operator, as it allows more precise control over the comparison:

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

This statement effectively compares some_variable to null and returns true if it's either null or undefined.

Notes

• The simplified notation is useful when checking variables that are assumed to be defined, such as optional arguments or object properties.
• Undefined global variables will always evaluate to undefined, while checking for them with typeof can be beneficial in certain scenarios.
• The Nullish coalescing operator (??) and Logical nullish assignment (??=) offer concise alternatives for setting default values in the presence of null or undefined values.