如何解决Uncaught TypeError：无法将undefined或null转换为对象

Question

This is my html. I want to calculate height in feet, cm, inch. When I enter height value then I want to convert it into inch and feet on key up and vise-versa. Please help me.

<form name=cminch method="post">
    <div>
        <label>feet</label>
        <input onkeyup="feetconvert()" name="Heightft">
        <label>inch</label>
        <input onkeyup="inchconvert()"  name="HeightIn">
        <label>Cm</label>
        <input onkeyup="cmconvert()"" name="Heightcm"> 
    </div>
</form>

This is my JavaScript function

function roundit(which){
    return Math.round(which*100)/100
}

function cmconvert(){
    with (document.cminch){
        Heightft.value = roundit(Heightcm.value/30.84);
        HeightIn.value = roundit(Heightcm.value/2.54);
    }
}
function inchconvert(){
    with (document.cminch){
        Heightcm.value = roundit(HeightIn.value*2.54);
        Heightft.value=roundit(HeightIn.value/12);
    }
}
function feetconvert(){
    with (document.cminch){
        Heightcm.value=roundit(Heightft.value*30.48);
        HeightIn.value=roundit(Heightft.value*12);
    }
}

Answer 1

Just copy and paste whole code in your file try running the same and let me know if you still getting the same error.

Check an example into JSFiddle

<form name="cminch" method="post">
    <div>
      <label>feet</label>
      <input onkeyup="feetconvert()" name="Heightft">
      <label>inch</label>
      <input onkeyup="inchconvert()"  name="HeightIn">
      <label>Cm</label>
      <input onkeyup="cmconvert()" name="Heightcm"> 
    </div>
</form>

<script>
    function roundit(which){
        return Math.round(which*100)/100
    }

    function cmconvert(){
        with (document.cminch){
            Heightft.value = roundit(Heightcm.value/30.84);
            HeightIn.value = roundit(Heightcm.value/2.54);
        }
    }
    function inchconvert(){
        with (document.cminch){
            Heightcm.value = roundit(HeightIn.value*2.54);
            Heightft.value=roundit(HeightIn.value/12);
        }
    }
    function feetconvert(){
        with (document.cminch){
            Heightcm.value=roundit(Heightft.value*30.48);
            HeightIn.value=roundit(Heightft.value*12);
        }
    }
</script>