索引访问与JavaScript中的变量

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);
} 

Answer 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.

Answer 2

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.