Trouble with basic JavaScript loop

Let's first simplify this a bit and expand to make it more legible:

var x = new Array();
var y = new Array(1, 1, 0, 3, 5);
for(let i = 0; i < y.length; i++){
    let key = y[i];
    x[ key ] = y[i];
}

On first evaluation, i === 0, so key === y[0] or 1. Now that we have the key, we will assign the key as a value to the x array at the index that was the number stored in your y array at index i. So x[y[0]] = y[0], therefor x[1] === 1.

Wrince, repeat.

You have to realize that the positions where data is put in Array X are taken from Array Y.

As Array Y only have the following numbers (it means, positions): 0, 1, 3, 5, the positions 2 and 4 will be undefined in the Array X

var i;
var x = new Array();
var y = new Array(1, 1, 0, 3, 5);
for (i = 0; i < y.length; i++) {
    console.log("Index taken from ARRAY Y where number will be put in ARRAY X: " + y[i]);
    x[y[i]] = y[i];
}

Because you are not incrementing it by the index. You are setting the index based on the value of the index of the y array.

So what you are doing in your loop is

x[1] = 1  // i = 0;
x[1] = 1  // i = 1;
x[0] = 0  // i = 2;
x[3] = 3  // i = 3;
x[5] = 5  // i = 4;

There is no value of 2 or 4, so those indexes were never defined, so it it is undefined.

What you want is

for (i = 0; i < y.length; i++) {
    x[i] = y[i];
}

Try to write down each iteration and see how x changes in successive steps.

START           //                 []
x[y[0]] = y[0]; // x[1] = 1;       [undefined, 1]
x[y[1]] = y[1]; // x[1] = 1;       [undefined, 1]
x[y[2]] = y[2]; // x[0] = 0;       [0, 1]
x[y[3]] = y[3]; // x[3] = 3;       [0, 1, undefined, 3]
x[y[4]] = y[4]; // x[5] = 5;       [0, 1, undefined, 3, undefined, 5]