To Execute or Not to Execute: When Do Parentheses Matter in JavaScript Function Calls?

When Calling JavaScript Functions: Parentheses Matter, or Not

When invoking a function in JavaScript, developers may wonder if omitting parentheses makes a difference. Let's explore this topic through a real-world example.

Consider the following code snippet:

window.onload = initAll();

Here, initAll() is a function without any parameters. If we call it with empty parentheses as shown above, the following occurs:

• initAll() is executed immediately, as if we called initAll().
• The return value of initAll() is assigned to the window.onload event handler. This behavior may not be desirable if initAll() is not intended to return a function.

Contrast this with the following code:

window.onload = initAll;

In this case, parentheses are omitted. As a result:

• initAll itself is assigned to the window.onload event handler, without being executed.
• initAll will be invoked automatically when the load event fires.

The key distinction is that omitting parentheses assigns the function reference, while using empty parentheses immediately executes the function and assigns its return value.

In summary, use empty parentheses to execute a function immediately and assign its return value. For assigning the function reference without execution, omit parentheses. This understanding empowers developers to write efficient and accurate JavaScript code.

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