Question

I am running this code and checking the log in firebug:

    var a = new Array();
var b = new Array();
for (i=0; i<2 ; i++){
    a.push(1);
    b.push(a);
    console.log("a", a);
    console.log("b", b);
};

The log shows these values:

a [1]
b [[1]]
a [1,1]
b [[1,1],[1,1]]

According to me the values should be:

a [1]
b [[1]]
a [1,1]
b [[1],[1,1]]

What am I doing wrong and how could I get the values that I want?

Answer 1

When you’re using .push to append a to the end of b, you’re passing a reference and not ByVal as an Array is an Object. This means that future changes to a are reflected in the as already in b.
What you need is to .slice a when .pushing to b.

var a = new Array(), b = new Array();
for (i=0; i<2 ; i++) {
    a.push(1);
    b.push(a.slice());
    console.log("a", a);
    console.log("b", b);
};
/*
a [1]
b [[1]]
a [1, 1]
b [[1], [1, 1]]
*/