for(var i=0;i<lines.length;i++) { var line = lines[i]; vs
var line; for(var i=0;i<lines.length;i++) { line = lines[i]; Do this two snippets have different performance. If yes, please explain why.
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2 Answers
I have run a benchmark on this and have found that there is pretty much no difference in performance.
If you would like to recreate it:
var lines = new Array(#BIG_NUMBER#); lines.fill(#BIG_OBJECT#); var a = (new Date()).getTime(); for(var i=0;i<lines.length;i++) { var line = lines[i]; } var b = (new Date()).getTime(); var line; for(var i=0;i<lines.length;i++) { line = lines[i]; } var c = (new Date()).getTime(); console.log("a:" + (b-a) + " b:" + (c-b)); This makes sense, as there is almost no difference, both cases will create a local object that will be stored, and will be reassigning it multiple times.
Comments
Your code will work differently in both the cases.
In the first one line is a global variable an will be accessible outside the for loop, which is not the case in second one.
So I don't think performance comparison makes any sense here!
Even then, declaring a variable again and again is unnecessary though I don't think it is a complex operation. But yes declaring it repetitively will deteriorate the performance.
4 Comments
jcubic
In the second one the variable also be global and accesible outside of for loop. In javascript there are no block scope only function scope.
void
@jcubic there is block level scope in ES6.
jcubic
OP ask about
var not let.
Andy
Even if you were right, you've got your examples mixed up.