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?? : Null Coalescing Operator

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Summary

Provide a fallback value when an expression evaluates to null or undefined.

Syntax

Precedence

Priority level in expression evaluation (1 = highest, 10 = lowest): 8

Evaluated after: ^, *, /, %, +, -, <, <=, >, >=, ==, !=

Evaluated before: &&, ||

Operands

Position Type Description
Left Any Expression that might be null
Right Any Fallback value if left is null

Returns

Type: Same as operands

Returns the left operand if it’s not null, otherwise returns the right operand.

Examples

Default User Name 

<h2>Welcome, !</h2>

Data (with name):

doc.Params["model"] = new {
    user = new {
        name = "John Doe"
    }
};

Output:

<h2>Welcome, John Doe!</h2>

Data (without name):

doc.Params["model"] = new {
    user = new {
        name = (string)null
    }
};

Output:

<h2>Welcome, Guest!</h2>

Missing Description 


  <div class="product">
    <h3></h3>
    <p></p>
  </div>

Data:

doc.Params["model"] = new {
    products = new[] {
        new { name = "Widget A", description = "High quality widget" },
        new { name = "Widget B", description = (string)null },
        new { name = "Widget C", description = "Premium widget" }
    }
};

Output:

<div class="product">
  <h3>Widget A</h3>
  <p>High quality widget</p>
</div>
<div class="product">
  <h3>Widget B</h3>
  <p>No description available</p>
</div>
<div class="product">
  <h3>Widget C</h3>
  <p>Premium widget</p>
</div>

Optional Contact Information 

<div class="contact-info">
  <p>Email: </p>
  <p>Phone: </p>
  <p>Address: </p>
</div>

Nested Property Access 

<p>City: </p>

Data (nested null):

doc.Params["model"] = new {
    user = new {
        address = (object)null
    }
};

Output:

<p>City: Unknown</p>

Default Values in Lists 


  <tr>
    <td></td>
    <td></td>
    <td>$</td>
    <td></td>
  </tr>

Data:

doc.Params["model"] = new {
    items = new[] {
        new { name = "Item 1", category = "Electronics", price = 99.99m, notes = "In stock" },
        new { name = "Item 2", category = (string)null, price = (decimal?)null, notes = (string)null }
    }
};

Output:

<tr>
  <td>Item 1</td>
  <td>Electronics</td>
  <td>$99.99</td>
  <td>In stock</td>
</tr>
<tr>
  <td>Item 2</td>
  <td>Uncategorized</td>
  <td>$0</td>
  <td>-</td>
</tr>

Chaining Multiple Defaults 

<p>Display Name: </p>

Default Image 


  <img src="" alt="" />

Notes

  • Returns left operand if not null/undefined, otherwise returns right operand
  • Very useful for providing default values
  • Can chain multiple null coalescing operators
  • Evaluates left-to-right when chained
  • Does not check for empty strings - only null/undefined
  • More concise than default
  • Similar to C# null-coalescing operator
  • Use ifError() function for error handling instead of null checking

Null vs Empty String

The ?? operator only checks for null/undefined, not empty strings:

<!-- Empty string is NOT null -->

<!-- If model.name = "", output is "" not "Default" -->

<!-- To handle empty strings, use a conditional -->
Default

See Also