Using multiple sets of brackets in Javascript to create a type of array?

Question

Just when I think I'm getting an understanding of the syntax of Javascript, I run into something like this...

I saw this code on the jqPlot plugin page.

what kind of array is [[]] in javascript? A nested array? I thought I understood the double brackets, but in the commented text, I don't understand why there is an extra set (question below).

$(document).ready(function(){
// Our data renderer function, returns an array of the form:
// [[[x1, sin(x1)], [x2, sin(x2)], ...]]
var sineRenderer = function() {
var data = [[]];
for (var i=0; i<13; i+=0.5) {
  data[0].push([i, Math.sin(i)]);
}
return data;
};

Why is there an extra set of brackets in this commented statement (the outermost set)?

[[[x1, sin(x1)], [x2, sin(x2)], ...]]

Thanks

[[],[],[]] is the syntax for an array that contains other arrays. Each inner array in your example contains a number and the sine function of that number. — Hunter McMillen
– Hunter McMillen, Commented Jun 20, 2012 at 16:56

Sarfraz · Accepted Answer · 2012-06-20 16:59:10Z

1

Since you are putting in arrays in your data array (which creates arrays inside an array), doing this would avoid that:

var data = [];
for (var i=0; i<13; i+=0.5) {
  data.push([i, Math.sin(i)]);
}

When you use [[]] and then do data[0], you are not utilizing outer array there which jumps in when you output it.

answered Jun 20, 2012 at 16:59

Sarfraz

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Amadan · Accepted Answer · 2012-06-20 16:57:07Z

1

It would be a single set of brackets if they said data.push. Since the 0-th element of the array is an array, they use data[0].push to stuff stuff in it.

Why they need it double-wrapped, no idea without more context.

answered Jun 20, 2012 at 16:57

Amadan

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2 Comments

Chris22 Over a year ago

yes, the "double-wrapped" is what I was wondering about. The expression doesn't seem to need three sets of brackets...

Chris22 Over a year ago

Maybe this example is better context. Please note the extra outer array (3 bracket sets): $.jqplot('chartdiv', [[[1, 2],[3,5.12],[5,13.1],[7,33.6],[9,85.9],[11,219.9]]]); Taken from jqplot.com/docs/files/usage-txt.html.

Bergi · Accepted Answer · 2012-06-20 17:00:13Z

1

Yes, it's a nested array. The data array contains an array to which other arrays with value and sine of the value are pushed.

AFAIK, the jQPlot plugin would use every array in data to print a new graph. You could for example do

function graph(fn, from, to, gap) {
    // creates an array with points
    var res = [];
    for (var i=from; i<to; i+=gap) {
        res.push([i, fn(i)]);
    }
    return res;
}

var data = [
   graph(Math.sin, 0, 13, 0.5),
   graph(Math.cos, 0, 13, 1)
];
...

answered Jun 20, 2012 at 17:00

Bergi

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Comments

Tina Bell Vance · Accepted Answer · 2012-06-20 17:01:59Z

1

Hunter is correct -- it's the syntax for an array containing other arrays. You could also look at doing this as an object, as illustrated here: http://www.quirksmode.org/js/associative.html.

answered Jun 20, 2012 at 17:01

Tina Bell Vance

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