JavaScript and Open Data

5Arrays

For a data scientist, handling arrays of tabulated data is a recurring task. The baseline, if using JavaScript, is to be able to do what we usually do with a spreadsheet and basic macrooperations. Thousands and thousands of tabulated data can be accessed freely on the Internet: public data for most countries, from international bodies, or free access private data. In Part 3, several applications will be discussed (e.g. French parliament election 2017).

JavaScript provides several built-in objects able to represent tabulated data, which we can access through an index: Array, of course, and TypedArray, Map, Set, Stringare other “Iterables”. They all share several features, such as the length property, but are named “array-like”. The list of arguments of a function, and a list of selected HTML DOM elements, are also “array-like” objects.

This chapter is devoted to the object “Array”.

An “array” is a set of ordered values, which we can access with a numeric index. This index starts at 0 not 1 (zero-based index).

There is no specific type for arrays: the operator typeof returns "object", but the static method Array.isArray(tab) can check if its argument is an array, hence tab inherits all the Array.prototype methods.

5.1. Handling arrays: creation and access to its elements

5.1.1. Creating an array with the array literal notation

The syntax of an “array literal” uses [square brackets] to delimit a list of elements, which can be primitive values, variable names, objects ...