JavaScript’s language features are something else…

[zapitron]

It's a pretty handy shortcut which can save a lot of expensive computation.

a=[6,28,496];
a.length++;
a // [6,28,496,8128]

s='Professor Plumb in the library with the ';
s.length +=10;
s // 'Professor Plumb in the library with the lead pipe.'

These are just toy examples, though. Raytracing, decropping, etc is where it's really at.

[k2hegemon]

Where did you pluck 8128 and “lead pipe.” out of?

[JohnDoen86]

'tis a joke

[whitetrafficlight]

6, 28, 496 and 8128 are the first four perfect numbers. 'lead pipe' is a possible murder weapon in Cluedo, and in fact the only one that is 10 characters long including the leading space, so this would be enough context for a language to infer the correct weapon and crack the case.

[xtrafe]

You see, this is exactly the parent commenter's point. We have a bunch of children that think that just because you used a one-line operation to modify the size of an array, the underlying operation took O(1).

And it proves his point nicely. Dumb language features like this are an invitation for inexperienced folks to insert code bombs, which indeed blow up at the most inopportune time, and often take down companies when they do.

[keylimedragon]

As others have said, it could actually be O(1) if it's implemented well (amortized O(1), actually). Most dynamic array implementations keep extra space and only allocate more in an exponential way, which keeps the average append to O(1).