I came across this, understood it but still did not know what javascript property it was:
var array = [];
array[1] = true;
array[5] = true;
console.log(array) #=> [true, true]
array[0] #=> undefined
array[1] #=> true
array[2] #=> undefined
array[5] #=> true
Can someone explain this for me? Thanks
Standard arrays in JavaScript aren't really arrays at all, and one effect of that is that they're inherently sparse. That is, an array can have empty slots in it.
That's what you're creating there. After your first three lines, you have an array with two entries in it, at indexes 1 and 5, and a bunch of completely empty slots (indexes 0, 2, 3, 4). Its length property would be 6. When you try to retrieve an element that doesn't exist from an array, you get the value undefined. (This is just a specific case of JavaScript's general behavior: If you try to retrieve an object property that doesn't exist, you get the value undefined.)
The output of console.log with a sparse array will vary depending on what the implementation of console.log does with them. The comments on the question suggest that there are various different ways the console may show the array. You might look at using console.log(array.join()) to get more consistent results. That would give you ,true,,,,true, because it shows blanks for array entries that don't exist (or that contain the value undefined, but in your case, they don't exist).
console.logwould have printed[ , true, , , , true ]. Please check again.[undefined, true, undefined, undefined, undefined, true].[1: true, 5: true]. Either way, someone should explain.[undefined × 1, true, undefined × 3, true]:-|