Javascript for loop in

Question

I have a for loop which goes through an array but I am trying to restrict it to break after the 2nd item.

var x="",i=0;
for (var x in marking;i=0;i++) {
 if (i==2){
 break;
 }
 index.push(marking[x]);
};

The console log error

SyntaxError: missing ) after for-loop control
 for (var x in marking;i=0;i++) {

Could someone please show me how to combine the

for (var x in marking)

and

for (i=0;i<10;i++)

Answer 1

If you need to break after 2nd item, have you considered following approach:

for (var i=0;i<2;i++) {
 index.push(marking[i]);
};

• Skwal: x doesn't exist in your example.

Answer 2

Simply put the i++ inside your for in loop

var i=0;
for (var x in marking) {
 if (i==2){
 break;
 }
 index.push(marking[x]);
 i++;
};

NB: You don't need to declare the x variable before the loop since you re-declare it inside of it.

• TimWolla: You should not use for (... in ...) to iterate over an Array: stackoverflow.com/questions/500504/…
• Skwal: It's probably not the best place to debate this, but I think as long as it's not misused, it's perfectly fine for use for in loops for Arrays as long as you don't mess up with the array prototype or change it on the fly. The link in your comment says the same.

Answer 3

edit

Come to think of it, you don't need a loop to do this at all. You can just take a slice.

If index already contains something, you can use concat:

index.concat(marking.slice(0,2));

If index has been declared already but is empty:

index = marking.slice(0,2);

This will add the first two items of marking to the end of index.

previous answer - potentially unsafe

1. There's no need to have var x in both places. In JavaScript, a declaration anywhere within a function means that the variable exists from the top of the function but is undefined until the line you have declared it. To help remember that, I normally put my var declarations at the top of the function.
2. Put ++i (or i++, or i+=1) in your loop.
3. (削除) Use === when comparing two things of the same type. (削除ここまで) == is slightly faster.

Like this:

var x,i=0;
for (x in marking) {
 if (i == 2){
 break;
 }
 index.push(marking[x]);
 ++i;
}

For conciseness you could even combine the increment and comparison, like:

var x,i=0;
for (x in marking) { 
 index.push(marking[x]);
 if (++i == 2){
 break;
 } 
}

Will break after the second item and only go through two iterations of the loop, rather than breaking on the third.

• Skwal: Comparing two integers is slightly faster with == than ===. In this case you know i is an integer so it's safe to use the ==. (Source: jsperf.com/double-vs-triple)
• TimWolla: You should not use for (... in ...) to iterate over an Array: stackoverflow.com/questions/500504/…

Answer 4

var i=0;
for (var x in marking) {
 if (i==2){
 break;
 }
 index.push(marking[x]);
 i++;
};

• TimWolla: You should not use for (... in ...) to iterate over an Array: stackoverflow.com/questions/500504/…