Benchmark: is variable not undefined? typeof variable !== "undefined" vs variable !== undefined

Tests:

typeof undefined is not "undefined"
typeof undefined !== "undefined"
typeof string is not "undefined"
typeof "not undefined" !== "undefined"
undefined is not undefined
undefined !== undefined
string is not undefined
"not undefined" !== undefined

Rendered benchmark preparation results:

Suite status: <idle, ready to run>

Previous results

Test case name	Result
typeof undefined is not "undefined"
typeof string is not "undefined"
undefined is not undefined
string is not undefined

Fastest: N/A

Slowest: N/A

Latest run results:

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Autogenerated LLM Summary (model gemma2:9b, generated one year ago):

LLMs can make mistakes. Check important info.

This benchmark tests different ways to check if a variable is defined in JavaScript.

**Options Compared:**

* **`typeof undefined !== "undefined"`:** This checks if the *type* of `undefined` is not equal to `"undefined"`. It's relying on the fact that `typeof undefined` returns `"undefined"`.
* **`typeof "not undefined" !== "undefined"`:** This checks if the *type* of a string literal ("not undefined") is not equal to `"undefined"`. 
* **`undefined !== undefined`:**  This directly compares `undefined` with itself, which seems counterintuitive but can be useful in some contexts.

**Pros and Cons:**

* **`typeof undefined !== "undefined"`:**
    * **Pro:** It's the common way to check for undefinedness. Most developers will understand it intuitively. 
    * **Con:** Can be less performant compared to other options, as it involves a type coercion process.

* **`typeof "not undefined" !== "undefined"`:** This test is designed to deliberately introduce unnecessary complexity. It's unlikely to be used in practice.

* **`undefined !== undefined`:**
    * **Pro:** Can be more performant than `typeof` based checks as it avoids type coercion. 
    * **Con:**  Intuitively confusing and less readable. Might not be obvious why this works.

**Other Considerations:**

* The benchmark results show that the fastest approach for checking if a variable is defined in JavaScript is likely to be one of the `typeof` based checks (assuming the code is correctly optimized).
* **Best Practice:** In most cases, using  `typeof undefined !== "undefined"` is a good practice because it's widely understood and generally performs well.

**Alternatives:**

* **Nullish Coalescing Operator (`??`)**: Introduced in ES2019, this operator provides a concise way to check for null or undefined values: `value ?? defaultValue`. For example: `myValue ?? "default value"` will return `"default value"` if `myValue` is null or undefined.
* **Optional Chaining (`?.`)**: Another modern feature (ES2020), this allows you to safely access properties of objects without worrying about potential `undefined` values: `object?.property`.

Let me know if you have any other questions!

Related benchmarks:

is variable not undefined? typeof variable !== "undefined" vs variable !== undefined (version: 0)

Comparing performance of: typeof undefined is not "undefined" vs typeof string is not "undefined" vs undefined is not undefined vs string is not undefined

Created: 5 years ago by: Guest

Jump to the latest result

typeof undefined is not "undefined"

typeof string is not "undefined"

undefined is not undefined

string is not undefined

Suite status: <idle, ready to run>

Fastest: N/A

Slowest: N/A

No previous run results

Autogenerated LLM Summary (model gemma2:9b, generated one year ago):