CODEWITHSUNDEEP
javascript
Aral Roca
Aral Roca

Reputation: 5909

How can do this function pure

I want to map an array, returning the same array except .created property, that I'm transforming from milliseconds to Date Object.

const posts = data.map((post) => {
    post.created = new Date(post.created); //milliseconds to Date Object
    return post;
})

This function do the result that I expected, but I don't like because the function inside is not pure, is mutating the object. How can be an alternative to do the same with pure function?

Thank you so much.

Upvotes: 1

Views: 77

Answers (3)

Davin Tryon
Davin Tryon

Reputation: 67296

You can achieve this using Object.assign (docs) inside the map:

const posts = data.map((post) => {
    return Object.assign({}, post, { created: new Date(post.created) });
})

This essentially clones the original post and then overrides the created property.

Upvotes: 2

Sylwester
Sylwester

Reputation: 48745

You can make copy without copying all of the attributes from the old object:

const posts = data.map((post) => { 
    const obj = Object.create(post); 
    obj.created = new Date(post.created); // shadow created with the new value
    return obj;
});

In practice it uses JavaScript inheritance scheme so that the new object only has created as it's own attribute and the rest is inherited from the original object.

Upvotes: 1

Lewis
Lewis

Reputation: 14866

I would provide both solutions.

Clone the array and its objects

data.map(post => Object.assign({created: new Date(post.created)},post));

The first argument of Object.assign is the target object. It's also the object that Object.assign returns.

Modify the existing array

for(let post of data){
    post.created = new Date(post.created); 
}

This method is, of course, faster and costs less resources since it doesn't need to initialize a new array and its objects then copy all objects' properties like the other does.

The choice is yours. Just make sure you know what you're doing.

Upvotes: 1

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