JavaScript TypeError in 2D array

Question

I'm trying to read value from a json into a 2D array

  var arr = [[],[]];
  var i = 0;
    var file = './test.json';
    jsonfile.readFile(file, function(err, objs) {
        let i = 0;
        objs.forEach(function (obj) {
            arr[i][0] = obj["outcount"];
            arr[i][1] = obj["gateID"];
            arr[i][2] = obj["timestamp"];
            arr[i][3] = obj["eventCode"];
            i = i + 1;
        });
        console.log(arr[2]);
    })

For i = 0 and i = 1, it works fine but on i = 2 it gives the error

TypeError: Cannot set property '0' of undefined

arr only contains 2 elements, that you get with arr[0] and arr[1] but when you try to get a 3rd, non-existant element with arr[2] you get undefined and then you try to access a property of undefined which gives you that TypeError — Lennholm
– Lennholm, Commented Jun 18, 2017 at 21:55
@MikaelLennholm How do I create a 2D array with no limit on elements? — Arpit
– Arpit, Commented Jun 18, 2017 at 21:56
See Musa's answer. Arrays don't have a set number of elements nor a maximum number of elements, you can add and remove from an array freely — Lennholm
– Lennholm, Commented Jun 18, 2017 at 22:00

Musa · Accepted Answer · 2017-06-18 22:03:15Z

2

Your array arr only has two items in it, the empty arrays.
So arr[0] is the first one and arr[1] is the second one that's all, arr[2] doesn't exist. If you had arr = [[],[],[]]; then arr[2] would work but you can see the problem there.
Best option is to create the sub-array before you use it.
Also, the forEach function provides a counter which can be used instead of creating your own.

var arr = [];
var file = './test.json';
jsonfile.readFile(file, function(err, objs) {
    objs.forEach(function (obj, i) {
        arr[i] = [];
        arr[i][0] = obj["outcount"];
        arr[i][1] = obj["gateID"];
        arr[i][2] = obj["timestamp"];
        arr[i][3] = obj["eventCode"];
        i = i + 1;
    });
    console.log(arr[2]);
})

edited Jun 18, 2017 at 22:03

answered Jun 18, 2017 at 21:55

Musa

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3 Comments

Lennholm Over a year ago

I would also suggest using the index provided by the forEach() function instead of creating your own counter

Arpit Over a year ago

I see javascript handles arrays differently than most other languages. The square brackets depict separate arrays instead of dimensions.

Lennholm Over a year ago

@Arpit Yes, arrays in Javascript are just objects with some dedicated syntactic sugar. There are no true arrays in JS.

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