Question

var arr = ["a", "b", "c",null,undefined];

delete arr[0];

for (var i = 0,j=arr.length; i < j; i++) {
    console.log(arr[i]);
    /*
    undefined
    b
    c
    null
    undefined
    */
}

for (var o in arr) {
    console.log(arr[o]);
    /*
    b
    c
    null
    undefined
    */
}

who can tell me why the output is different?

Answer 1

you have access by key;

array index is key of array object

using delete keyword, remove object key
because

delete arr[0]; // key "0" deleted!

ok?

and see this

var arr = ["a", "b", "c",null,undefined];
delete arr[0]; // key "0" deleted!

// access by i,  with key : 0
for (var i = 0,j=arr.length; i < j; i++) {
    console.log(i);
    /*
    0 
    1 
    2 
    3 
    4 
    */
}

// access by object key, without key : 0
for (var o in arr) {
    console.log(o);
    /*
    1
    2
    3
    4 
    */
}​​​​​​​​​​

good luck!