How can I fill the empty slots of a sparse array with values (e.g. undefined)?
let array = ['a',,,,'z']
// Should return
['a', undefined, undefined, undefined, 'z']
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2 Answers
This will fill it up :
let array = ['a',,,,'z'];
Array.from(array);
// Returns ['a', undefined, undefined, undefined, 'z']
5 Comments
pilchard
Nice and clean, but spread syntax will be significantly more performant than Array.from().
[...array]
VLAZ
@pilchard I get 1529 ops/sec for spread syntax and 1514 ops/sec for Array.from. I wouldn't call that "significantly" more performant. It's 0.7% better. Re-ran the tests a few times and the results were similar each time. The widest difference I got was 1536 ops/sec for spread syntax vs 1494 ops/sec for Array.from. Which is a 2.8% increase. I'd still hardly call it "significant".
pilchard
@VLAZ I initially ran the benchmarks on Safari which shows a whopping 200% increase from Array.from to Spread. But in Chrome I get results closer to yours.
Ben Dak
@VLAZ, which tests do you run and how do you run them, to get those numbers?
VLAZ
@BenDak the ones linked by pilchard : measurethat.net/Benchmarks/Show/13243/0/arrayfrom-vs-forof-vs scroll down and you'd see a "Run tests" button: i.imgur.com/9hvQNN8.png
You can spread the array and you will get undefined in the previously empty slots
let array = ['a',,,,'z']
const result = [...array];
console.log(result);
// ['a', undefined, undefined, undefined, 'z']
for(let i = 0; i < array.length; i++) {
test("array ", array, i);
test("result", result, i);
}
function test(name, arr, num) {
console.log(`${num} in ${name}: ${num in arr}`);
}.as-console-wrapper {
max-height: 100% !important;
}