Strange array behaviour in NodeJS

Question

I have the following code written for NodeJS:

/* server.js */
'use strict';

const http = require('http'),
    url = require('url');
    METHODS = ['GET','POST','PUT','DELETE'],
    _routePathIndex = Array.apply(null, Array(METHODS.length)).map(() => {return []}),
    _routeMethodIndex = _routePathIndex.slice(),
    _server = http.createServer();

_server.on('request', (req, res) => {
    let parsed = url.parse(req.url),
        methodIndexVal = METHODS.indexOf(req.method),
        PathIndexVal = _routePathIndex[methodIndexVal].indexOf(parsed.pathname);

    _routeMethodIndex[methodIndexVal][PathIndexVal](req, res);
});

module.exports = _init();

function _init(){
    let rs = { listen: _listen };
    METHODS.forEach( (val,i) => {
        rs[val.toLowerCase()] = function(route, handler){
            _routePathIndex[i].push(route);
            _routeMethodIndex[i].push(handler);
        };
    });

    return rs;
};

function _listen(port, callback){
    _server.listen(port, callback);
}

To test this out I have the I have a very simple script:

/* server.test.js */
var app = require('./server.js');
app.get('/', (req,res) => { console.log(req, res); });
app.listen(3000, () => { console.log('listening at port', 3000) });

The strangeness begins on line 2 of server.test.js which executes the following code block in server.js, I added the comments to display the values of both _routePathIndex and _routeMethodIndex.

...
        rs[val.toLowerCase()] = function(route, handler){
            /* _routePathIndex:    [ [], [], [], [], ]
               _routeMethodIndex:  [ [], [], [], [], ] */
            _routePathIndex[i].push(route);

            /* _routePathIndex:    [ ['/'], [], [], [], ]
               _routeMethodIndex:  [ ['/'], [], [], [], ] */
            _routeMethodIndex[i].push(handler);


            /* _routePathIndex:    [ ['/', [Function]], [], [], [], ]
               _routeMethodIndex:  [ ['/', [Function]], [], [], [], ] */
        }; 
...

My question is, why is the array acting as tho there are referenced to each other?

At first, I thought maybe it was the .slice() that was making the reference but I debunked that by running the following script in the same environment:

var a = [], b = a.slice();
a.push(1);
console.log(a,b); // [1] [0]

Another thing is when I don't do the .slice() trick and refactored the code to as such

...
    _routePathIndex = Array.apply(null, Array(METHODS.length)).map(() => {return []}),
    _routeMethodIndex = Array.apply(null, Array(METHODS.length)).map(() => {return []}),

the strange referencing behaviour is gone and the code works Perfect!

For added info I'm working with node -v : v5.4.1.

Also I tried to clone the array using [].concat(_routePathIndex) but it still had that weird behaviour

Array.apply(null, Array(METHODS.length)).map(() => {return []}) seems (to me) a convoluted and unreadable way of writing Array(METHODS.length).fill([]) — jcaron
– jcaron, Commented Feb 20, 2016 at 17:46

georg · Accepted Answer · 2016-02-20 17:50:21Z

5

slice only does a shallow copy, that is _routePathIndex and _routeMethodIndex are different, but their elements are the same. Consider this simplified example:

a = [[],[]];
b = a.slice();
    
b[0].push(1);
document.write(a)

Get a picture:

edited Feb 20, 2016 at 17:50

answered Feb 20, 2016 at 17:43

georg

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2 Comments

jkris Over a year ago

can't accept answer too soon, SO rules, anyway what did you use to for the cool pic?

georg Over a year ago

@jkris: merribithouse.net/arrows - developed this for my own use a while ago.

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