I have been watching PluralSight's Rapid JavaScript Training by Mark Zamoyta and I came across this. He showed these two examples. I've been trying to wrap my head around it, but still could not understand.
How is it able to capture the length of the entries after the array was created using new Array() method, seeing that it returned a blank array []. If it's blank like this [], shouldn't it return -1?
var entries = [1,2,3,4,5];
entries.length
=> 5
entries
=> [ 1, 2, 3, 4, 5 ]
var entries = new Array(5);
entries.length
=> 5
entries
=> []
var myArray = new Array(5);
When you define an array by passing the constructor an integer like above, memory is allocated for 5 slots in the array. If you examine the array, you will find:
console.log(myArray[1]);
=> undefined
console.log(myArray.toString);
=> ,,,,
As you can see, there are indeed five elements in the array, each of them undefined. So your array isn't "blank."
It is probably bad practice to initialize an array in this manner, as there just isn't a good use case for it. Pushing to the array will yield:
myArray.push("value");
console.log(myArray.toString);
=> ,,,,,value
...which is never what you want. I would advise initializing the array like below and forget that passing an integer to the constructor is even an option:
var myArray = [];