This IS five iterations, but we are starting at 0, not one. Our loop will start at 0, and run until it equals “4”. So, since our array’s “length” property is “5”, our “len” variable is equal to “4”. In Example # 2, we create a variable who’s value is one LESS than the length of our array. Now this is the most important point, and it’s what explains the “off” reference: the length of the array is always one higher than the index of the last array element because the array indexes are zero-based, but the length property is one-based. So, even though the array has a length of 5, the first element has an index of 0, and the last element (the 5th element) has an index of 4. The console.log() statement reflects this as well because the “length” property of this array is “5” (i.e. In Example # 1, we have an array with five elements. This is because, again, JavaScript arrays are zero-based.
But (and here is the point where many get confused), if a JavaScript array has four elements, the last element has an index of “3”. If a JavaScript array has four elements, then that array’s “length” property will have a value of “four”.
So, a JavaScript array with one element will have a “length” of “1”. The JavaScript array length property is given in a one-based context. This is not unusual in computer programming languages. In other words, the index value of the first element in the array is “0” and the index value of the second element is “1”, the third element’s index value is “2”, and so on. This means that JavaScript starts counting from zero when it indexes an array.
The “length” property of a JavaScript array is a very helpful tool, but why is array lengthĪrrays in JavaScript are zero-based.
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