Possible Duplicate:
Difference between using var and not using var in JavaScript
sometime, I saw people doing this
for(var i=0; i< array.length; i++){ //bababa }
but I also see people doing this...
for(i=0; i< array.length; i++){ //bababa }
What is the different between two? Thank you.
Always declare JavaScript variables with var , let , or const . The var keyword is used in all JavaScript code from 1995 to 2015. The let and const keywords were added to JavaScript in 2015. If you want your code to run in older browser, you must use var .
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In Javascript, it doesn't matter how many times you use the keyword “var”. If it's the same name in the same function, you are pointing to the same variable. This function scope can be a source of a lot of bugs.
"Optional" is perhaps an unfortunate choice of words, as var is optional in the sense that an assignment statement can be successfully interpreted with or without it, but not optional in the case where you want to explicitly declare a variable in local scope.
The var
keyword is never "needed". However if you don't use it then the variable that you are declaring will be exposed in the global scope (i.e. as a property on the window
object). Usually this is not what you want.
Usually you only want your variable to be visible in the current scope, and this is what var
does for you. It declares the variable in the current scope only (though note that in some cases the "current scope" will coincide with the "global scope", in which case there is no difference between using var
and not using var
).
When writing code, you should prefer this syntax:
for(var i=0; i< array.length; i++){ //bababa }
Or if you must, then like this:
var i; for(i=0; i< array.length; i++){ //bababa }
Doing it like this:
for(i=0; i< array.length; i++){ //bababa }
...will create a variable called i
in the global scope. If someone else happened to also be using a global i
variable, then you've just overwritten their variable.
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