Array values gets mutated while doing iteration with forEach and map

Question

I am trying to implement some logic inn which i would like to have the array as it is, but when i tried to do with forEach and map the values in info array get mutated.

var info = [{"date":"today","location":["a","b","c","d","e","f","g","h"]},{"date":"yesterday","location":["a","b","c"]},{"date":"tomorrow","location":["a","b","c"]}]

var showItems = [];
var restrictItems= [];
info.forEach(res =>{
                if(res.location.length && restrictItems.length<5)
                {
                    let slicedLocation = res.location.slice(0,5-restrictItems.length);
                    slicedLocation.map((item, i) => {
                        
                            restrictItems.push(item);
                        
                    });
                    res.location = slicedLocation;
                    showItems.push(res);
                }
            })

console.log("showItemsshowItems",showItems);// here i am getting the output as [{"date":"today","location":["a","b","c","d","e"]}] which is expected
console.log("info",info)// here i am not getting the original value of info array(f,g,h) is missing in First array.
[{"date":"today","location":["a","b","c","d","e"]},{"date":"yesterday","location":["a","b","c"]},{"date":"tomorrow","location":["a","b","c"]}]

Please help here, thanks in advance.

topdev87 · Accepted Answer · 2021-02-11 14:23:09Z

You should clone info variable.

var cloned = JSON.parse(JSON.stringify(info))

And use cloned variable to keep info.

var info = [{"date":"today","location":["a","b","c","d","e","f","g","h"]},{"date":"yesterday","location":["a","b","c"]},{"date":"tomorrow","location":["a","b","c"]}]
var cloned = JSON.parse(JSON.stringify(info))

var showItems = [];
var restrictItems= [];
cloned.forEach(res =>{
                if(res.location.length && restrictItems.length<5)
                {
                    let slicedLocation = res.location.slice(0,5-restrictItems.length);
                    slicedLocation.map((item, i) => {
                        
                            restrictItems.push(item);
                        
                    });
                    res.location = slicedLocation;
                    showItems.push(res);
                }
            })

console.log("showItemsshowItems",showItems);
console.log("info",info)

sonEtLumiere · Accepted Answer · 2021-02-11 14:24:20Z

0

You can clone your original array to prevent mutation by using slice() method:

let clonedInfo = info.slice();

answered Feb 11, 2021 at 14:24

sonEtLumiere

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Long Nguyễn Thành · Accepted Answer · 2021-02-11 14:29:54Z

0

First item of array: you slice it (0,5) (it means abcde), and assign it to 'slicedLocation', then you assign res.location = slicedLocation;. That is why your 'location' of first item contain only abcde

answered Feb 11, 2021 at 14:29

Long Nguyễn Thành

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