Why Does Array.prototype.push Return the Updated Array Length Instead of More Informative Data?

Why Array.prototype.push Returns the New Length Instead of More Useful Data

Introduced in ECMA-262, 3rd Edition, Array.prototype.push has consistently returned a Number representing the updated array length. However, questions arise about why it was designed this way instead of providing potentially more valuable information, such as references to appended items or the modified array itself.

Historical Context

To understand the reasoning behind this design decision, we delve into the history of push and its counterparts in Perl. Added in 1997 to Netscape 4's JS1.2, these methods were inspired by their Perl counterparts.

Initially, push in JS1.2 conformed to Perl 4 by returning the last element added. However, in JS1.3, it shifted to align with Perl 5's convention of returning the new array length. This change was reflected in the jsarray.c source code, which distinguishes between Perl 4 and Perl 5 behavior based on the JS version.

Reasons for Returning the Array Length

This historical insight suggests that the decision to return the array's new length was aligned with the Perl 5 standard, which was influential in JavaScript's early development. Furthermore, returning the length may have been considered a clear and consistent way to indicate the result of the push operation, without introducing potential confusion or ambiguity.

Alternative Considerations

Returning references to appended items or the modified array itself has its merits. It could provide direct access to the newly added elements or allow for chained operations. However, these alternatives also present potential drawbacks:

• Returning references to individual items could make it more verbose to retrieve all appended items, especially for large arrays.
• Returning the modified array introduces the potential for accidental mutation, as it makes it harder to differentiate between the original array and the updated version.

Conclusion

The Array.prototype.push method's return value of the new array length may not provide the most extensive information, but its design remains consistent with Perl's conventions and offers a clear indication of the operation's result. Alternative approaches have their advantages but also potential drawbacks that may have influenced the choice made in the early days of JavaScript.

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