The null coalescing operator (??) provides a concise way to assign default values when dealing with null or undefined. 1. It returns the left operand if it is not null or undefined; otherwise, it returns the right operand. 2. Unlike the logical OR (||) operator, ?? only triggers the fallback for null or undefined, preserving falsy values like "", 0, or false. 3. Common use cases include default parameters, configuration settings, API responses, and user input handling. 4. Chaining is supported: value1 ?? value2 ?? value3 returns the first non-null value. 5. Language differences exist—C# supports nullable types, PHP suppresses undefined variable errors, and JavaScript strictly checks null and undefined. 6. Operator precedence requires parentheses when combining with && or ||. Using ?? leads to safer, more predictable code by distinguishing missing values from valid falsy ones, making it superior to || for default value assignment when intent is to handle only null or undefined, and this clarity enhances code readability and reliability.

The ?? operator—known as the null coalescing operator—is a clean, modern way to handle null values in languages like C#, PHP, JavaScript (as of ES2020), and others. It simplifies default value assignment when dealing with potentially null or undefined expressions, making code more readable and less error-prone than traditional checks.

What the Null Coalescing Operator Does

The ?? operator returns the left-hand operand if it’s not null or undefined; otherwise, it returns the right-hand operand.

string name = userInput ?? "Guest";

In this example:

• If userInput is not null, name gets the value of userInput.
• If userInput is null, name defaults to "Guest".

This is functionally equivalent to:

string name = userInput != null ? userInput : "Guest";

But it's shorter, clearer, and avoids repeating the variable.

Why It’s Better Than Ternary or OR (||)

A common alternative—especially in JavaScript—has been the logical OR (||) operator:

const name = userInput || "Guest";

But there's a key difference: || falls back on any falsy value (like "", 0, false, NaN), not just null or undefined.

const userInput = "";
const name = userInput || "Guest"; // returns "Guest" — maybe not intended!

With ??, only null or undefined trigger the fallback:

const name = userInput ?? "Guest"; // returns "" — preserves empty string

So if you want to distinguish between null and valid but falsy values, ?? is safer and more precise.

Practical Use Cases

Here are common scenarios where ?? shines:

• Default function parameters (in languages that support it)
• Configuration fallbacks
• API response handling
• User input sanitization

For example, in C#:

int timeout = config.Timeout ?? 30;

Or in PHP:

$theme = $_SESSION['theme'] ?? 'light';

You can also chain multiple ?? operators:

string displayName = user.ProvidedName ?? user.Username ?? "Anonymous";

This reads naturally: “Use the provided name if available, otherwise the username, otherwise call them Anonymous.”

Caveats and Language Differences

• In C#, ?? works with nullable value types and reference types.
• In PHP, it returns the right operand if the left is null (doesn’t trigger warnings if the variable doesn’t exist, unlike ??=).
• In JavaScript, ?? is only for null and undefined—it respects 0, false, and "".

Also, operator precedence matters. ?? has lower precedence than most operators, so wrap it in parentheses when mixing with && or ||:

const result = (value ?? defaultValue) && isActive;

Bottom Line

The null coalescing operator isn’t just syntactic sugar—it encourages safer, more intentional handling of null values. By focusing only on null and undefined, it avoids the pitfalls of truthiness-based fallbacks. Once you start using ??, you’ll likely find yourself reaching for || less often in default-value scenarios.

Basically: use ?? when you mean “if this is missing, use that”—not “if this is falsy.” It’s a small operator with a big impact on code clarity.

The above is the detailed content of The Null Coalescing Operator (??): A Modern Approach to Handling Nulls. For more information, please follow other related articles on the PHP Chinese website!