Why Does Chrome Display \"Undefined x 1\" in the Debugger?

Demystifying "Undefined x 1" in JavaScript

In the context of a JavaScript program, encountering "undefined x 1" can be puzzling. This message typically appears within Chrome's debugger and signifies a newly introduced feature.

Understanding "undefined x 1"

"Undefined x 1" denotes an uninitialized element in an array or array-like object. This is a recent visual enhancement in Chrome, replacing the previous practice of displaying multiple "undefined" values. For example, instead of showing "[undefined, undefined, undefined,...]", it succinctly indicates "[undefined x 100]" for an array with 100 uninitialized elements.

Uninitialized Elements

Uninitialized elements are placeholders within an array that have yet to be assigned a value. In JavaScript, arrays are dynamic and can be resized dynamically. When an array is created, elements beyond the initial size are uninitialized and contain the special value "undefined."

Arguments Array

In the context of a function, the "arguments" array is an array-like object that collects the arguments passed to the function. While elements of this object can be accessed like array elements (e.g., arguments[0]), they are not true array elements and cannot be deleted. Attempting to delete them results in undefined.

Example

Consider the following snippet:

function foo(x) {
  console.log(arguments[0]);
}

foo(); // Logs undefined

foo(1); // Logs 1

In the first call to foo(), no arguments are passed, resulting in "undefined x 1" in the debugger. In the second call, an argument is provided, and the usual "undefined" is printed.

Conclusion

"Undefined x 1" in Chrome's debugger indicates uninitialized elements in arrays or array-like objects. Understanding this can aid in debugging and manipulating such objects effectively.

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