Effective JavaScript Item 49 takes precedence over the for loop for array traversal, not for. In loop

this series as effective JavaScript 's reading notes.

Can you see what the last average is for this code?

var = [98, scores, Max, Max, 89];var, max = 0;for (var score in scores) {total + = score;} var mean = Total/scores.length;mean; // ?

by calculation, the final result should be the .

but don't forget toFor : InchLoop, is traversed forever isKey, but notvalue, the same is true for arrays. Therefore the aboveFor : Inchin the Loopscorenot expected.98, Abouta series of values, but0,1A series of indexes.

So you might think that the final result is:

(0 + 1+ ... + 6)/7 =

But the answer is also wrong. Another key point is that for: the type of the key in the in loop is always the string type, so the + operator here performs the concatenation of the string actually:

finally get the Total is actually a string 00123456 . This string is converted to a numeric type after the value is 123456, and then divided by the number of elements 7, the final result is obtained:17636.571428571428

so, for array traversal, use the standard for Cycle Best:

var scores = [98, n, a, 0, 89];var total = 0;for (var i =, n = scores.length; i < N.; i++) {Total + = Score S[i];} var mean = Total/scores.length;mean; 88

after all, for this for loops, developers are no longer familiar with it. The index variable i is never treated as a value. The standard for Loop also guarantees the order of the loops, ensuring that this is important for the arithmetic operations of floating-point numbers.

in the above code, a small optimization is also applied. is to calculate the length of the array as the loop boundary before the loop begins. When you do not add / remove element operations to the array itself in the For loop , you can slightly improve performance so that the length of the collection is not queried for Each loop.

Summarize:

1. when iterating through an array, use the standard for Loop instead of using the For : in Loop.
2. Consider pre-saving the length of the array where necessary to improve performance.

Effective JavaScript Item 49 takes precedence over the for loop for array traversal, not for. In loop