Node: Float numbers not pushed into array.

[SkeLLLa]

In two words: float numbers not pushed into array if in push function shift is called.

Version: 8.5.0
Platform: linux x86_64

class FixedArray extends Array {
  constructor(length) {
    super(length);
    this.maxLength = length;
  }

  push(...args) {
    console.log('what to push', args);
    super.push.apply(this, args);
    console.log('array after push', this);
    if (this.length > this.maxLength) {
      console.log('what is shifted?', super.shift.apply(this));
    }
  }
}

var ddd = new FixedArray(30);
ddd.push(1);
ddd.push(2);
ddd.push(3);
ddd.push(0.4);
ddd.push(0.5);
ddd.push(0.6);
console.log(ddd);

Result:

what to push [ 1 ]
array after push FixedArray [ <30 empty items>, 1, maxLength: 30 ]
what is shifted? undefined
what to push [ 2 ]
array after push FixedArray [ <29 empty items>, 1, 2, maxLength: 30 ]
what is shifted? undefined
what to push [ 3 ]
array after push FixedArray [ <28 empty items>, 1, 2, 3, maxLength: 30 ]
what is shifted? undefined
what to push [ 0.4 ]
array after push FixedArray [ <27 empty items>, 1, 2, 3, 0.4, maxLength: 30 ]
what is shifted? undefined
what to push [ 0.5 ]
array after push FixedArray [ <27 empty items>, 1, 2, 3, 0.5, maxLength: 30 ]
what is shifted? undefined
what to push [ 0.6 ]
array after push FixedArray [ <27 empty items>, 1, 2, 3, 0.6, maxLength: 30 ]
what is shifted? undefined
FixedArray [ <27 empty items>, 1, 2, 3, maxLength: 30 ]

Here's the code that stopped working in 8.5.0, but it was working in 8.4.0.

There's a class FixedArray that basically overrides push method. When I try to push float value into such array it pushes only once, however it should be pushed every second.

Another strange thing is if you change it to ddd.push(5); it works normally and integer value is pushed into this array every second.

If you remove shift() from a push, then all works also normally.

PS: In chrome versions from 60 to 63 all works normally.

[vsemozhetbyt]

Simplified case:

'use strict';

var ar = new Array(5);

ar.push(1);
console.log(ar);

ar.shift();
console.log(ar);

ar.push(2);
console.log(ar);

ar.shift();
console.log(ar);

ar.push(0.3);
console.log(ar);

ar.shift();
console.log(ar);

[ <5 empty items>, 1 ]
[ <4 empty items>, 1 ]
[ <4 empty items>, 1, 2 ]
[ <3 empty items>, 1, 2 ]
[ <3 empty items>, 1, 2, 0.3 ]
[ <3 empty items>, 1, 2 ]

[targos]

This is fixed in V8 6.1.

[targos]

/cc @nodejs/v8

[SkeLLLa]

@targos then this is related to https://github.com/nodejs/node/pull/15393

[vsemozhetbyt]

Even more simplified case:

var ar = [, , 0.1];

ar.shift();
console.log(ar);

[ <2 empty items> ]

So the issue is: Array.prototype.shift() skips empty slots till the first float number and shifts this number instead?

[SkeLLLa]

@vsemozhetbyt I don't think so, because is you look into my example there were some integers before floats and they were not shifted. Also:

var a = [,,,0.4,1,2,3,0.5];
a.shift()
console.log(a) // it will remove 0.5, not 0.4

[vsemozhetbyt]

Yes, it seems for arrays with an initial hole and a non-SMI number, Array.prototype.shift() becomes Array.prototype.pop().

const a = [ , 2**31, 1 ];
a.shift();
console.log(a); // [ <1 empty item>, 2147483648 ]

[targos]

The commit message here looks like it could be fixing this: https://github.com/v8/v8/commit/81778aaf72d0daded8470afdd14203a70707117b

[andrasq]

a variant of Array.prototype.pop(), it pops but doesn't return the value (returns undefined)

[targos]

https://github.com/v8/v8/commit/81778aaf72d0daded8470afdd14203a70707117b was actually merged to V8 6.1 so this issue is fixed on master.

[bnoordhuis]

v8.7.0 was released with V8 6.1, which has the fix. I'll close this out.