js fill array than replace with value

Question

My goal is start with an array with null values than if an other array has entries replace accordingly just to do en example

const a = [...[null,null,null, null], ...[1,2,3,4]]

I want an array [1,2,3,4]

at start the second array could be also empty like

const a = [...[null,null,null, null], ...[]]

or

const a = [...[null,null,null, null], ...[1]]

and so on ....

const a = [null,null,null, null];
const b = [1,2,3];
const c = a.map((item,i)=> b[i] ? b[i] : a[i])

Is there a better way ?

What is your expected result for the second and third examples you've given? Or when you have [...[null,null,null, null], ...[1,2]]? — Terry
– Terry, Commented Apr 21, 2020 at 7:45

Yone · Accepted Answer · 2020-04-21 14:06:07Z

1

You could take Object.assign with an array as target and get the values updated which are at the same index/key.

This works with sparse arrays as well.

const a = [null, null, null, null];
const b = [1, 2, 3, , 5];

// creating new, rather than mutating
const c = Object.assign([], a, b); 

console.log(c); // [1, 2, 3, null, 5]

edited Apr 21, 2020 at 14:06

Yone

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answered Apr 21, 2020 at 7:36

Nina Scholz

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Comments

MING WU · Accepted Answer · 2020-04-21 08:03:58Z

0

Check it out. just a simple for loop. I use Number.isNaN to do the validation. You can use other validations if you need to validate other kind of values.

    const a = [null,null,null, null];
    const b = [1,2,3];

    let c=[...a]
    for (let i = 0; i < b.length; i++) {
        if(!Number.isNaN(b[i])){
            c[i]=b[i]
        }
    }

This is the output:

[ 1, 2, 3, null ]

answered Apr 21, 2020 at 8:03

MING WU

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