无法进入我的for循环（JavaScript）

Question

I am trying to iterate over the variable pieces , but somehow it is not triggering. In the function below, pieces.length turns 4, but piecesNew turns [] - moreover console.log(i) is never reached.

What is happening?

self.reworkPieces = function(pieces) {

        var piecesNew = [];

        console.log(pieces.length)

        for (var i; i <= pieces.length; i++) {

            console.log(i)

            piecesNew[i] = {
                c: pieces[i].r,
                r: pieces[i].c,
                p: i
            }

        }

        console.log(piecesNew)

        return piecesNew;

    }; 

Answer 1

undefined is not less than or greater than any number.

var i; // undefined
i < 10; // false
i > 10; // false

You simply need to make sure i is equal to 0 .

You also want i to never be equal to pieces.length so use < instead of <= (the last index of an array is always length - 1 , eg an array of length 3 has indexes 0, 1 and 2 – the first index is always 0, not 1)

So:

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

Answer 2

define value for variable i, var i = 0

self.reworkPieces = function(pieces) {

        var piecesNew = [];

        console.log(pieces.length)

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

            console.log(i)

            piecesNew[i] = {
                c: pieces[i].r,
                r: pieces[i].c,
                p: i
            }

        }

        console.log(piecesNew)

        return piecesNew;

    }; 

Answer 3

self.reworkPieces = function(pieces) {

    var piecesNew = [];

    console.log(pieces.length)

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

        console.log(i)

        piecesNew[i] = {
            c: pieces[i].r,
            r: pieces[i].c,
            p: i
        }

    }

    console.log(piecesNew)

    return piecesNew;

}; 

Answer 4

Change:

 for (var i; i <= pieces.length; i++)

To:

 for (var i = 0; i < pieces.length; i++)

Answer 5

Initialize var i = 0 initially then it will work

self.reworkPieces = function(pieces) {

            var piecesNew = [];

            console.log(pieces.length)

            for (var i = 0; i <= pieces.length - 1; i++) {

                console.log(i)

                piecesNew[i] = {
                    c: pieces[i].r,
                    r: pieces[i].c,
                    p: i
                }

            }

            console.log(piecesNew)

            return piecesNew;

        };