Taking a JavaScript object with 4 properties:
function Object() {
this.prop1;
this.prop2;
this.prop3;
this.prop4;
}
var obj = new Object();
I use a for(in) loop to inspect each property since I don't know the number or name of the properties:
for(property in obj) {
var prop = obj[property];
}
However I would like to process the properties starting with the last (prop4 in this example). I suppose I would like a reverse-for-in-loop.
How can I do this?
Thanks, Jack
Adding: The object I am referring to is the one returned from JSON.parse. The properties seem to be consistently ordered. There is no keys() method.
To reverse for loop in Python just need to read the last element first and then the last but one and so on till the element is at index 0. You can do it with the range function, List Comprehension, or reversed() function.
Backwards for loop is faster because the upper-bound (hehe, lower-bound) loop control variable does not have to be defined or fetched from an object; it is a constant zero. There is no real difference. Native loop constructs are always going to be very fast.
To use the forEach() method on an array in reverse order:Use the slice() method to get a copy of the array. Use the reverse() method to reverse the copied array. Call the forEach() method on the reversed array.
A for (x in y)
does not process the properties in any specific order so you cannot count on any desired order.
If you need to process the properties in a specific order, you will need to get all the properties into an array, sort the array appropriately and then use those keys in the desired order.
Using ES5 (or an ES5 shim), you can get all properties into an array with:
var keys = Object.keys(obj);
You could then sort them either in standard lexical order or sort with your own custom function:
keys.sort(fn);
And, then you could access them in your desired order:
for (var i = 0; i < keys.length; i++) {
// process obj[keys[i]]
}
The ECMAScript standard does not define an order to iteration for for in
loops. You will want an array, if your datatypes are to be sorted.
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With