javascript - for loop efficiency problem

Question

for (var i = 0, len = source.length; i < len; i )

Is

better than

for (var i = 0; i < source.length; i )

Is it efficient?

Answer 1

The first way of writing avoids calculating length every time and is more efficient than the second way of writing. (The number of loops is not many. In fact, there is not much difference between the two, but it is recommended to use the first way of writing)

Answer 2

The second typeevery time it loops will get the length of the source,

The first method only obtains the value of the source length once and caches it in a variable. In the future, the value is obtained from the variable every time,

The first one is more efficient.

Answer 3

Practice is the only criterion for testing understanding

var a = new Array(100).fill(0);

var a = new Array(1000).fill(0);

var a = new Array(10000).fill(0);

You can see from the results that the first way of writing has slightly better performance, but the first way will consume extra memory. Although front-end memory is free, for me personally, there is no difference between the two methods