JavaScript for loops iterate over each item in an array. Creating an Array Using an array literal is the easiest way to create a JavaScript Array. So we eventually get: So, our array of four elements has indexes from 0 to 3. The items property is accessed as follows: data.items The value is an array, to access its second element, we have to use bracket notation: data.items[1] This value is an object and we use dot notation again to access the name property. JavaScript arrays are zero based, which means the first item is referenced with an index of 0. An array can hold many values under a single name, and you can access the values by referring to an index number. As we saw, arrays can have several dimensions, which means that an array element can contain an array, whose elements can contain arrays, etc. By default the indexing of every JavaScript array starts from 0. Calling Array(N) results in a RangeError, if N is a non-whole number whose fractional portion is non-zero. The problem most likely is that the value is undefined at some point in the components lifecycle (you're probably loading it asynchronously). The following example illustrates this behavior. You are going to need be be defensive in the way that you reference your property. It's not like an array where you can access a value by index ("pick the first value"). The Basic For Loop. Remember that the index values start at 0, not 1. Here is an example that demonstrates $.ajax() and shows you how to access the returned values in an array. As we can see data is an object, hence we can access its properties using dot notation. let arr = Array (9.3) // RangeError: Invalid array length. I recommend using $.ajax() and specifying the dataType as JSON, or using $.getJSON(). Looping Through an Array. I want to create an array whose indexing starts from 1 instead. The syntax to access an array member It sounds like the data structure is confusing :-) If you have an object with unknown/random property names, you can't really access a specific property without knowing the name. We can loop through the entirety of the array with the for keyword, taking advantage of the length property. – Strille Oct 3 '13 at 9:04 An “indexed” array is one where the index must be an integer, and you access its elements using its index as a reference. I ask because when you find yourself trying to circumvent one of the most basic characteristics of a programming language, odds are good there's a better way to do what you're trying to do. If your code needs to create arrays with single elements of an arbitrary data type, it is safer to use array … This means that array indexes start at 0 and go up to the number of elements, minus 1. It would help to know what your AJAX request looks like. 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