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Undefined when trying to access object in Javascript

I’m creating a javascript program that prompt users to input coordinates (x,y) of 2 points of each line to do further functions.

    const numOfLines = parseInt(prompt("Please enter number of lines: "));

    //point object
    function Point(x, y)
    {
        this.x = x;
        this.y = y;
    }

    let firstPoints = new Array(numOfLines);
    let secondPoints = new Array(numOfLines);

    for(let i = 0; i < numOfLines; i++)
    {
        let firstPointMessage = "Please enter the first point's coordinates x, y of line " + (i+1);

        // substring input to add (x,y) to an array
        let firstPoint = prompt(firstPointMessage).split(",");

         //add point(x,y) to firstPoints array
        let fp = new Point(parseInt(firstPoint[0]) ,parseInt(firstPoint[1]));
        firstPoints.push(fp);

        let secondPointMessage = "Please enter the second point's coordinates x, y of line " + (i+1);
        let secondPoint = prompt(secondPointMessage).split(",");
        let sp = new Point(parseInt(secondPoint[0]) ,parseInt(secondPoint[1]));
        secondPoints.push(sp);
    }

    while(num < numOfLines)
    {
        document.write(firstPoints[num].x);
        ....further action
        num++;
    }
</script>

Error Cannot read properties of undefined (reading ‘x’) occurred for line document.write(firstPoints[num].x);

And firstPoints[num] actually has values when I tried to debug, but I just can’t get the value out of it to use for further action. What should I do?
Thanks

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2

Answers


  1. 0

    while(num < numOfLines) – to me seems you’re using a loop on the num variable but you never declared num as a variable anywhere in the code.

    I’m a noob but I just noticed that so apologies if that sounds dumb.

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  2. 0

    You can use your i var to populate exact array cell with your wanted value.

    I see that you are defining the length of the array first but you don’t need to do that, there really is no benefit, this is not C++, you have like 4GB of memory to work with.

    for(let i = 0; i < numOfLines; i++)
    {
        let firstPointMessage = "Please enter the first point's coordinates x, y of line " + (i+1);

        // substring input to add (x,y) to an array
        let firstPoint = prompt(firstPointMessage).split(",");

         //add point(x,y) to firstPoints array
        let fp = new Point(parseInt(firstPoint[0]) ,parseInt(firstPoint[1]));
        firstPoints[i]=fp); // assign the value to an explicit array position

        let secondPointMessage = "Please enter the second point's coordinates x, y of line " + (i+1);
        let secondPoint = prompt(secondPointMessage).split(",");
        let sp = new Point(parseInt(secondPoint[0]) ,parseInt(secondPoint[1]));
        secondPoints[i] = sp);// assign the value to an explicit array position
    }
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