使用for循环语句了解数组迭代

Question

So i have this simple script that populates a variable with an array. I then use a function with a for loop to iterate trough the array to get it's index values.

function printAllArrayValues(array) {
    for (var i = 0; i < array.length; i++) {
        var c;
        c += array[i];
    }
    return c;
}
var colorArray = ["brown", "blue", "green"];
alert(printAllArrayValues(colorArray));

The function returns a string containing all the array values but the first value = undefined. See fiddle: http://jsfiddle.net/vcyum/

Why is that?

Answer 1

An simpler solution :

var colorArray = ["brown", "blue", "green"];
alert(colorArray.join('')); // "brownbluegreen"
alert(colorArray.join(',')); // "brown,blue,green"

Answer 2

The initial value of the c variable is undefined . The next line c += 'stuff' adds the string 'stuff' to the value in c . Since the initial value of c is undefined , it is cast to a string, resulting in 'undefined' , so the value of c is now 'undefinedstuff' .

Your code can be fixed like this:

function printAllArrayValues(array) {
    var c = '';
    for (var i = 0; i < array.length; i++) {        
        c += array[i];
    }
    return c;
}
var colorArray = ["brown", "blue", "green"];
alert(printAllArrayValues(colorArray));

or simpler:

var colorArray = ["brown", "blue", "green"];
alert(colorArray.join(''));

Answer 3

To fix your bug, you can change

for(var i = 0; i < array.length; i++){
    var c;

to

var c = '';
for(var i = 0; i < array.length; i++){

so that you don't add to undefined (which is transformed to "undefined" in the string concatenation) at the first iteration.

But you could also replace your whole function with colorArray.join('') .

Answer 4

Within your loop you declare c , but assign nothing to it. The variable thus has the 'value' undefined . Now if you add a string to it, undefined is boxed into a string and the string you add is appended to that.

Anyway: no need for a loop here: use colorArray.join(''); for the same result.