var, loop in javascript, cannot understand the output, error

Question

    var arr = [];
    for(var i=0; i<5; i++){
        arr[i] = function(){
            return i;
        };
    }
    document.write(arr[1]());

the output is 5, as I expected

but when i added i++ between return i; and the end of the for loop, like the code below,

    var arr = [];
    for(var i=0; i<5; i++){
        arr[i] = function(){
            return i;
        };
        i++;
    }
    document.write(arr[1]());

screen shows the error, Uncaught TypeError: arr[1] is not a function

i expected that output should be 6, but i cannot understand why.

Becuase after your first loop i will be 2. I suggest using some debugging tools to look at your variables or add some console.log()'s to your code to understand what's going on next time. — George
– George, Commented Jun 29, 2017 at 12:36
Because you're increasing the index by 2, array[0], array[2], array[4]... — dloeda
– dloeda, Commented Jun 29, 2017 at 12:36
Are you also expecting all the functions within arr to output the same value? That's what will happen. — evolutionxbox
– evolutionxbox, Commented Jun 29, 2017 at 13:21

Pointy · Accepted Answer · 2017-06-29 12:36:41Z

6

Because you've got two increments for i, the assigned elements of the array will be 0, 2, and 4. Element 1 is skipped.

The first assignment happens when i is 0. Then, i is incremented to 1 at the end of the loop, and then again to 2 in the third clause of the for loop header. So the next assignment is for element 2.

answered Jun 29, 2017 at 12:36

Pointy

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4 Comments

jwkoo Over a year ago

I can accept your answer 11mins later, anyway, thank you

evolutionxbox Over a year ago

Also, without a closure won't all the functions within arr will output 5.

Pointy Over a year ago

@evolutionxbox yes that's true but apparently the OP already knows that; it's mentioned in the question.

evolutionxbox Over a year ago

Good point! I missed the last line, except the output won't be 6 it'd be 5.

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