TransitArrayMap behind-the-scenes conversion can disrupt forEach loops

[nasonfish]

Hi folks,

I'm running into a problem with a loop where the amount of accesses on an ArrayMap exceeds the threshold specified in the types file, and thus a loop terminates prematurely.

Consider the following example:

const transit = require("transit-js");

const my_map = transit.map();
// initialize a map with 10 key-value pairs
my_map.set(1,2);
my_map.set(3,4);
my_map.set(5,6);
my_map.set(7,8);
my_map.set(9,10);
my_map.set(11,12);
my_map.set(13,14);
my_map.set(15,16);
my_map.set(17,18);
my_map.set(19,20);

function loop(v,k){
    for(let i = 0; i < 100; i++){
        my_map.get(1);
    }
    console.log("k, v", k, v);
}
my_map.forEach(loop);
// prints: k, v 1 2

Without the extraneous accesses of the map, this loop should print out all 10 of the key-value pairs which exist in the map. However, it is my understanding that some under-the-hood conversion is going on, which interrupts the forEach loop prematurely before it is able to complete, and thus this code only completes one forEach iteration before completing and terminating.

I see that transit/types.js, line 635, defines a conversion that my map matches the criteria for-- the _entries list is converted to a backingMap (since I have defined a small map with many accesses), and the _entries field is cleared in the process. I am assuming this is the condition which is causing a problem for my usage of the map.

I was wondering if you all had any recommendation on how to handle this-- if I should not be using a forEach that also uses the outer map, or if I should be making a copy of the map if I plan to use it in a forEach loop, or if this is a bug which needs to be handled on Transit's end.

Thanks!