Javascript: forEach() loop to populate an array - closure issue
Solution 1
var heavy_fruits = [];
myfruit = {}; // here's your object
fruits.forEach(function(item,index) {
if ( item.weight > 100 ) {
myfruit ["name"] = item.name;
myfruit ["weight"] = item.weight; // you modify it's properties
}
heavy_fruits.push(myfruit); // you push it to the array
});
You end up with an array [myfruit, myfruit, myfruit, myfruit]
.
Now if you modify myfruit
anywhere in the code, the change will be visible in every single occurence of myfruit
. Why?
Because you are modifying the referenece to the object. In this example, your array stores just copies of your object. And if you change one of it, every single one will change, because they are all references.
To fix this with each iteration you should be creating a new object and then doing some stuff on it.
BTW, as a matter of fact, your if
could just be like this:
if ( item.weight > 100 ) {
heavy_fruits.push(item); // if `item` only has `name` and `weight` properties
}
Solution 2
fruits.forEach(function (item, index) {
if (item.weight > 100) {
myfruit = {};
myfruit["name"] = item.name;
myfruit["weight"] = item.weight;
heavy_fruits.push(myfruit);
}
});
Related videos on Youtube
nadir
Updated on June 04, 2022Comments
-
nadir almost 2 years
Let's say we have an array of objects like:
var fruits = [ {name:"banana", weight:150},{name:"apple", weight:95},{name:"orange", weight:160},{name:"kiwi", weight:80} ];
I want to populate a "heavy_fruits" array with items from the "fruits" array above which weight is > 100. Here is my code:
var heavy_fruits = []; myfruit = {}; fruits.forEach(function(item,index) { if ( item.weight > 100 ) { myfruit ["name"] = item.name; myfruit ["weight"] = item.weight; } heavy_fruits.push(myfruit); });
However it shows: name:"orange", weight:160 name:"orange", weight:160 name:"orange", weight:160 name:"orange", weight:160
I know this is an issue with mixing closures and loops... but I read an article (http://zsoltfabok.com/blog/2012/08/javascript-foreach/) explaining that I would avoid this kind of issue using a forEach loop instead of the classic for loop.
I know I can use array methods like filter(), etc. but I'm asking that on purpose since I'm actually having troubles with a much bigger function that I cannot expose here... So I tried to summarize and simplify my issue description with "fruits".
-
Tushar over 7 years
myfruit
is referencing to same object. Movemyfruit = {};
in theforEach
callback. And I'll suggest to usefilter
asvar heavy_fruits = fruits.filter(f => f.weight > 100);
. -
Tushar over 7 yearsIt's not closure issue, it's about referencing same object.
-
Admin over 7 yearsBasically what you are doing is mutating object's properties and because the array is in fact storing references to the same object, when you change one of the object's property, the change is visible in each reference.
-
Redu over 7 yearsAmongst all array methods
forEach
has got <= 2% use case. For this particular case you should use eitherfilter
orreduce
. -
Kate over 4 yearsI was having the same issue as the original poster, and the thing that fixed it for me was the first comment above by Tushar. The variable where the empty object is declared must be inside the
forEach
.
-
-
Tushar over 7 yearsPlease edit with more information. Code-only and "try this" answers are discouraged, because they contain no searchable content, and don't explain why someone should "try this". We make an effort here to be a resource for knowledge.
-
Tushar over 7 yearsThis is not different from the code in question. Except, this'll give array of two oranges than four.
-
Gangz over 7 yearsheavy_fruits.push(myfruit); was out of the if scope ..that is the problem
-
Gangz over 7 yearsThat is a code problem ..I don't find any other issues with that
-
nadir over 7 yearsThank you so much to everyone who took time to help me to find the solution: Albzi, Tushar, Adrian and Ganga. However Adrian provided the solution + the clear explanation of my issue + why it does not work + how I can fix it. I've struggling with that issue for days! Regards