Javascript - For Loop Key Undefined Only For 1st Iteration?

Why does this always print UNDEFINED for the first iteration of array[j]?? var array = [1,2,3,4,5] for (i in array) { console.log('array[i]:' +array[i]); for (j=i+1; j<

Solution 1:

That's because i contains a string value, not a number.

Setting j to i+1 makes it "0"+1, not 0+1, so that is "01". As there is no item in array with the key "01" you get undefined back.

When j++ is executed, it will convert the "01" into the number 1 and increase it, so that loop will run as expected to the end.

For the next iteration in the outer loop, you get j = "11", and when that is compared to the length of the array it will skip the inner loop. For the rest of the iterations in the outer loop, the inner loop will be skipped as j will start outside of the array.

Solution 2:

When you use the foreach syntax on the array, it is actually pulling the index of the array into the i variable as a string value. performing j = i + 1 sets the j variable to the string value 01, for which there is no corresponding value at that key in array so it returns undefined.

After the first iteration in the sub-loop, j++ will convert it to an integer and return the proper value at the index of the array.

See this question for more information about the foreach syntax you are using.

Solution 3:

It's because you're using a for...in loop over an array. When you use for...in, array indexes are viewed as hash keys and are therefore treated as strings. So when you begin your second loop and you set j to i + 1, you have really set j to "01". Since there is no array["01"] you get undefined for the first iteration. Upon the second iteration, you convert j back to a number by setting it to length - 1 and can therefore use it to access array indexes. Try this example:

var array = [1,2,3,4,5]
for (i in array) {

    console.log(typeof i);
    console.log("array[i]:" +array[i]);

    for (j=i+1; j<array.length-1;j++) {

        console.log(j);
        console.log("array[j]:"+ array[j]);
    }
}

The way you fix this is by NOT using for...in loops with arrays. Just stick with normal for loops and, of course, while and do loops where situation requires.

Solution 4:

The Rest of the Story

This is a late entry in the answer race.

Use for..of

for ( var i of array) automatically coerces the Array "indexes" to numbers - so the OP's problem is fixed simply by replacing "in" with "of"

Don't use for..in

JS documentation says

Note: for...in should not be used to iterate over an Array where the index order is important.

Why? All the current answers say the problem is that the "indexes" are really strings. How is that? We all know the "for loop" uses numbers and that works.

JS Arrays are not arrays but regular JS objects

A JS Array is actually a plain old JS object in disguise - Key:Value pairs where the key (property name) is always a string. An Array does not have real indexes! The "indexes" are just properties who's names are stringed-numbers, so "0", "1", etc. There are no numeric indexes. But you can say Array is a special case of JS objects - it uses invalid names (starts with a digit) and JS allows us to write as if Array actually has indexes. Just another of the many, many Javascript WTFs.