javascript - array.map is mutating my original array - Stack Overflow

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I am trying to update a field in an object in my array.

My object (simplified) looks something like this:

{
  "readableStructure": [
    {
      "ID": "1",
      "Name": "Name 1",
      "Description": "Adding, updating or removing users",
      "IconName": "fas fa-users-cog"
    },
    {
      "ID": "2",
      "Name": "Name 2",
      "Description": "Views overview",
      "IconName": "fas fa-street-view"
    },
    {
      "ID": "3",
      "Name": "Name 3",
      "Description": "Improvements to apps",
      "IconName": "fas fa-broadcast-tower"
    },
    {
      "ID": "4",
      "Name": "Name 4",
      "Description": "Managing badges",
      "IconName": "fas fa-id-badge"
    },
    {
      "ID": "5",
      "Name": "Name 5",
      "Description": "Art",
      "IconName": "fas fa-hand-holding-heart"
    }
  ]
}

and I call it like this:

prepareRepositories(this.props.primarySearchRepositories);

I can have several similar repos and I loop through them. And for each item in the array readableStructure I am trying to add a property called QSString using the following code.

prepareRepositories(repositories) {

    const repos = repositories.map((repo, index) => {

        const searchableColumns = ['Name', 'Description'];
        const newRepo = [];
        newRepo[0] = {};
        newRepo[0].readableStructure = [];

        newRepo[0].readableStructure = repo[0].readableStructure.map((item) => {
            item.QSString = this.getSearchableString(item, searchableColumns) || 'blaha';
            return item;
        });

        return newRepo;
    });

    return repos;
}

getSearchableString = (base, searchableColumns) => {
    let searchables = [];

    Object.keys(base).forEach((key) => {
        if ((searchableColumns.indexOf(key) > -1) && (base[key] !== null) && (typeof base[key] !== 'object')) {
            searchables.push(base[key]);
        }
    });

    const searchableString = searchables.join(' ');
    return searchableString;
}

However, when I do this the original object is modified and the QSString property get set to the same thing for every repo.

What am I doing wrong?

I am trying to update a field in an object in my array.

My object (simplified) looks something like this:

{
  "readableStructure": [
    {
      "ID": "1",
      "Name": "Name 1",
      "Description": "Adding, updating or removing users",
      "IconName": "fas fa-users-cog"
    },
    {
      "ID": "2",
      "Name": "Name 2",
      "Description": "Views overview",
      "IconName": "fas fa-street-view"
    },
    {
      "ID": "3",
      "Name": "Name 3",
      "Description": "Improvements to apps",
      "IconName": "fas fa-broadcast-tower"
    },
    {
      "ID": "4",
      "Name": "Name 4",
      "Description": "Managing badges",
      "IconName": "fas fa-id-badge"
    },
    {
      "ID": "5",
      "Name": "Name 5",
      "Description": "Art",
      "IconName": "fas fa-hand-holding-heart"
    }
  ]
}

and I call it like this:

prepareRepositories(this.props.primarySearchRepositories);

I can have several similar repos and I loop through them. And for each item in the array readableStructure I am trying to add a property called QSString using the following code.

prepareRepositories(repositories) {

    const repos = repositories.map((repo, index) => {

        const searchableColumns = ['Name', 'Description'];
        const newRepo = [];
        newRepo[0] = {};
        newRepo[0].readableStructure = [];

        newRepo[0].readableStructure = repo[0].readableStructure.map((item) => {
            item.QSString = this.getSearchableString(item, searchableColumns) || 'blaha';
            return item;
        });

        return newRepo;
    });

    return repos;
}

getSearchableString = (base, searchableColumns) => {
    let searchables = [];

    Object.keys(base).forEach((key) => {
        if ((searchableColumns.indexOf(key) > -1) && (base[key] !== null) && (typeof base[key] !== 'object')) {
            searchables.push(base[key]);
        }
    });

    const searchableString = searchables.join(' ');
    return searchableString;
}

However, when I do this the original object is modified and the QSString property get set to the same thing for every repo.

What am I doing wrong?

• javascript
• arrays
• object

Share Improve this question Follow asked Jun 5, 2018 at 9:24 WinterWinter 2,51522 gold badges2323 silver badges3030 bronze badges 3

• Have you tried debugging? – Jorge.V Commented Jun 5, 2018 at 9:27
• item.QSString = this.getSearchableString ... -- this statement causes the issue. You're modifying the object in-place. What did you expect otherwise? – 31piy Commented Jun 5, 2018 at 9:27
• I am using react, hence calling the function getSearchableString with this. – Winter Commented Jun 5, 2018 at 9:30

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2 Answers 2

Sorted by: Reset to default 9

Array.map does not change the original array, i.e. creates a new Array. However, any object in the Array continues to hold the same reference, hence, any change in object even inside map function will update the object in original array.

You need to create a copy for the object.

item = JSON.parse(JSON.stringify(item))

using this wont mutate the original object

JSON.parse(JSON.stringify(your_object))

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