Indexed access vs variable in javascript

Question

At various places, an element from array is being used at index. Instinct says replace it with a variable pointing to that element and use it. But is there any other difference than for better readability/management?

Example 1:

if (cols[i]["Type"] === 4) {
    this.someFunc(cols[i]["Name"], cols[i]["Value"].VarA, cols[i]["Value"].VarB);
}

Example 2:

var col = cols[i];
if (col["Type"] === 4) {
    this.someFunc(col["Name"], col["Value"].VarA, col["Value"].VarB);
}

@Tushar For that kind of optimization, the compiler will likely take care of it anyway. There's probably 0 runtime difference. — Joseph Marikle
– Joseph Marikle, Commented Apr 12, 2016 at 15:27
@JosephMarikle other than the first pass, trivial in this case but worth noting. — Jared Smith
– Jared Smith, Commented Apr 12, 2016 at 15:31

8protons · Accepted Answer · 2016-04-12 15:31:42Z

1

It would seem to me that referencing the value directly via variable is faster than referencing the value via array element:

https://jsfiddle.net/cLf7k35n/

var test = 4;
var myArray = [0, 1, 2, 3, 4];

console.time("array_reference");
if (myArray[4] === 4) {
  console.log(myArray[4]);
}
console.timeEnd("array_reference");

console.time("variable_reference");
if (test === 4) {
    console.log(test);
}
console.timeEnd("variable_reference");

Check the console for the timers. In the specific, non-complex example I made, the array reference seemed to be at least 1.2 milliseconds slower.

answered Apr 12, 2016 at 15:31

8protons

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1 Comment

Yahya Over a year ago

Forked your fiddle to include variable assignment and ran the tests. On average, it was 3 times faster on average to do via variable. jsfiddle.net/p0cj3tzg

Community · Accepted Answer · 2017-05-23 11:50:36Z

1

Example 2 will not need to do multiple array lookups so will be slightly faster. That being said, the JIT will most likely hoist that out for you.

In my opinion Example 2 is more readable and thus easier to maintain, so I would go with that.

Also as R3tep has stated, you can use col.Type and col.Value.VarA to improve readability further.

edited May 23, 2017 at 11:50

CommunityBot

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answered Apr 12, 2016 at 15:29

Joey Ciechanowicz

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1 Comment

Yahya Over a year ago

Your answer is relevant too, but the other answer provided an example to test. Thanks for your answer.

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