ISOMITTED Function in Excel 365 – Complete Guide

The ISOMITTED function is a new and specialized function available in Excel 365 that works exclusively within LAMBDA functions. It’s designed to check if an argument has been omitted when the LAMBDA function is called.

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📘 What is ISOMITTED in Excel?

ISOMITTED checks whether a specific parameter in a LAMBDA function was provided or left out when the function was called.

🔧 Syntax:

=ISOMITTED(argument)

• argument — A parameter defined in a LAMBDA.
• Returns TRUE if the argument is omitted, FALSE if provided.

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🧠 Why is it Useful?

It allows you to:

• Define optional parameters in your custom functions.
• Create default values when a parameter is not supplied.
• Add dynamic behavior depending on whether a user provided an input.

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✅ Example 1: Optional Discount Argument

Let’s define a function that calculates the total price after an optional discount.

=LAMBDA(price, discount,
    IF(
        ISOMITTED(discount),
        price,
        price - price * discount
    )
)(100)

💡 Since discount is omitted, it returns 100 — the original price.

But:

=LAMBDA(price, discount,
    IF(
        ISOMITTED(discount),
        price,
        price - price * discount
    )
)(100, 0.2)

💡 Returns 80 after applying the 20% discount.

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✅ Example 2: Creating a Named Function

You can also create a reusable function:

1. Go to Formulas > Name Manager > New
2. Name: SmartDiscount
3. Refers to:

=LAMBDA(price, discount,
    IF(ISOMITTED(discount), price, price - price * discount)
)

Now you can use:

=SmartDiscount(200)        → returns 200  
=SmartDiscount(200, 0.1)   → returns 180

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🛑 Limitations

• Can only be used inside a LAMBDA function
• Not available outside that context
• Only works in Excel 365 and Excel for the Web

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🎯 Use Cases

• Building reusable Excel mini-apps
• Creating optional inputs in custom functions
• Creating smarter calculators with defaults

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🎓 Want to Learn More Excel 365 Advanced Features?

If you want to explore LAMBDA, ISOMITTED, LET, and more dynamic Excel tools, check this out:

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• Excel 365-exclusive functions
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• Real-life examples and automations
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