Replace item or update it from Arraylist in a for in loop in node.js

Question

Let's say I have an array of objects, something like:

{'field1':value1, 'fiedl2':value2, 'field3':value3} ....

{'field1':value4, 'fiedl2':value6, 'field3':value1}

I want each object of the array to be changed, so

for(let item of items){
   //update item so it does not contain the old values but new calculated ones, so
   item.fieldA=item.field1+item.field2;
   item.fieldB=item.field3
}

So in the end, my array should have objects like:

{'fieldA':valueA, 'fieldB':valueB}

I am using node and I am trying to see how I can simply update or replace each object with its updated version with new properties and values

Youre just missing an delete item.field1 and youre done... — Jonas Wilms
– Jonas Wilms, Commented Jul 21, 2017 at 18:21
what is item.value1, did you mean item.field1? Where is your array of objects, I just see an object? — James
– James, Commented Jul 21, 2017 at 18:25

Jonas Wilms · Accepted Answer · 2017-07-21 18:40:51Z

1

It may be worth considering to simply create a new array:

items=items.map(before=>({
 fieldA:before.value1+before.value2,
 fieldB:before.value3
}));

Or pre ES6:

items=items.map(function(before){
 return {
    fieldA:before.value1+before.value2,
     fieldB:before.value3
 };
 }));

Through that, the original items are garbagge collected ( may be easier then deleteing all unused properties), and the syntax is quite nice.

And its may useful to use object destructuring:

items=items.map(
  ({value,value2,unused,...rest})
  =>
  ({result:value+value2,...rest})
);

The upper code deletes unused and replaces value and value2 with their addition stored as result, all the other parameters are copied too.

edited Jul 21, 2017 at 18:40

answered Jul 21, 2017 at 18:22

Jonas Wilms

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5 Comments

Jonas Wilms Over a year ago

@alin youre missing a return { ... } . If the brackets inside the arrow functions are enclosed by (), they arent a block statement anymore but rather an object literal.

Alin Over a year ago

Thank you very much I am new to js and missed it. Indeed now it works!

Jonas Wilms Over a year ago

@alin youre welcome ;) i admit that its quite confusing.

Alin Over a year ago

I like to use the pre ES6 for functions because I am familiar with java and this makes the move back and forth more easy. Probably I'll have to get into the => one day.

Jonas Wilms Over a year ago

@alin java is focused on readability and js on putting some stuff together very fast , i usually prefer the old function syntax if others need to read it ;)

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