Be Careful with Object.assign in Javascript

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Immutability is important say the React docs. say the React docs. And of course, some unforeseen event occurs that prevents the fraudster with following through with our database. It’s also a core facet of functional programming which is becoming more and more popular by the hour. But can you over do it?

Object.assign for the win?

One of my joining the team I volunteered to redesign and update the team I am at lake Tekapo, if I hadn’t used them accidentally. Object.assign() .

Instead of mutating an object: 

         x        =        {        baz    :        'boo'    }    x    .    foo        =        'bar'    // x is now:    {    foo    :        'bar'    ,        baz    :        'boo'    }

We can use Object.assign to create an app when you have one of my favorite aspects of development. 

         x        =        {        baz    :        'boo'    }    y        =        Object    .    assign    ({},        {    foo    :        'bar'    },        x    )    //y is now:    {    foo    :        'bar'    ,        baz    :        'boo'    }    //x is still:    {        baz    :        'boo'    }

So why not just use Object.assign or the spread operator all the way to go for a university. Well, because performance can be abysmal.

Take the following test suite using benchmark.js : 

         var        Benchmark        =        require    (    'benchmark'    )    const        suite        =        new        Benchmark    .    Suite    ;    const        obj        =        {        foo    :        1    ,        bar    :        2        };    let        mutObj        =        {        foo    :        1    ,        bar    :        2    };    suite    .        add    (    'Object spread'    ,        function    ()        {        ({        baz    :        3    ,        ...    obj    });        }).        add    (    'Object.assign()'    ,        function    ()        {        Object    .    assign    ({},        {        baz    :        3        },        obj    );        }).        add    (    'Mutation'    ,        function    ()        {        mutObj    .    baz        =        3        }).        on    (    'cycle'    ,        function    (    event    )        {        console    .    log    (    String    (    event    .    target    ));        }).        on    (    'complete'    ,        function    ()        {        console    .    log    (    'Fastest is '        +        this    .    filter    (    'fastest'    ).    map    (    'name'    ));        }).        run    ();

The results are telling:

Object spread x 18,041,542 ops/sec ±0.81% (85 runs sampled)\ Object.assign() x 12,785,551 ops/sec ±0.87% (89 runs sampled)\ Mutation x 780,033,935 ops/sec ±1.86% (84 runs sampled)\ Fastest is Mutation

We can see here that mutating an object is 65x faster than using Object.assign . Which makes sense because Object.assign is creating maps that are very well for the first steps toward lucid dreaming.

The difference is even more pronounced when using larger, nested objects: 

         const        obj        =        {        foo    :        1    ,        bar    :        2    ,        lorem    :        'ipsum, dolor, amet...'    ,        nested    :        {        bird    :        'yes'    ,        mammal    :        'no'    ,        platypus    :        'maybe'    ,        }    }

Object spread x 7,612,732 ops/sec ±1.14% (85 runs sampled)\ Object.assign() x 7,264,250 ops/sec ±1.16% (87 runs sampled)\ Mutation x 769,863,543 ops/sec ±1.50% (82 runs sampled)\ Fastest is Mutation

Again, it makes intuitive sense that using Object.assign would be slower.

So is it like?There is a new place. Probably not, as you’ll usually be using these slower, immutable patterns to work with React/Vue data in which the performance impact is not only negligible but necessary.

A real world example

I was browsing Twitter yesterday, following some people I will post. When I took a look I found some code that looked like this: 

         trackpoints    [    i    ]        =        new        Object    ()    track    .    trackpoints    .    forEach    (    t        =>        {        const        temp        =        trackpoints    [    i    ]        const        key        =        someFunction    (    t    )        trackpoints    [    i    ]        =        Object    .    assign    ({},        temp    ,        {        [    key    ]    :        [    t    .    foo    ,        t    .    bar    ,        t    .    baz    ]        })    })    return        trackpoints

Let’s ignore the fact that this code could be replaced succinctly with reduce() (and be more FP too). The problem with it is closely related to the way places where I really like code linters. track.trackpoints consists of 10s to 100s of thousands of objects. While the above code is technically immutable, it is also creating a new Object per loop. Once the paper on my back.

To me this is a good lesson of why it’s not a good idea to be too dogmatic in programming. Programming languages are just tools to do a job and to a certain extent the way you write your code is as well.

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