Check if Number entered is correct By ID - JavaScript

Would like to know how to check true and false and in return give error message if checked and the number is incorrect..

<input name="student1" type="text" size="1" id="studentgrade1"/>
<input name="student2" type="text" size="1" id="studentgrade2"/>
<input name="student3" type="text" size="1" id="studentgrade3"/>

so here we have 3 inputbox , now i would like to check the result by entering number into those inputbox.

studentgrade1 = 78

studentgrade2 = 49

studentgrade3 = 90

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So If User entered wrong number e.g "4" into inputbox of (studentgrade1) display error..

same for otherinputbox and if entered correct number display message and says.. correct.

http://jsfiddle.net/JxfcH/5/

OK your question is kinda unclear but i am assuming u want to show error if the input to the text-box is not equal to some prerequisite value.

here is the modified checkGrade function

function checkgrade() {
    var stud1 = document.getElementById("studentgrade1"); 
    VAR errText = "";    
    if (stud1.exists() && (parseInt(stud1.value) == 78){return true;}
    else{errText += "stud1  error";}
    //do similiar processing for stud2 and stud 3.
    alert(errText);

}

See demo →

I think this is what you're looking for, though I would recommend delimiting your "answer sheet" variable with commas and then using split(',') to make the array:

// answers
var result ="756789";

// turn result into array
var aResult = [];
for (var i = 0, il = result.length; i < il; i+=2) {
    aResult.push(result[i]+result[i+1]);
}

function checkgrade() {
    var tInput, 
        msg = '';

    for (var i = 0, il = aResult.length; i < il; i++) {

        tInput = document.getElementById('studentgrade'+(i+1));

        msg += 'Grade ' + (i+1) + ' ' +
            (tInput && tInput.value == aResult[i] ? '' : 'in') +
            'correct!<br>';

    }

    document.getElementById('messageDiv').innerHTML = msg;
}

See demo →

Try this http://jsfiddle.net/JxfcH/11/

function checkgrade() {

var stud1 = document.getElementById("studentgrade1");   
var stud2 = document.getElementById("studentgrade2");  
var stud3 = document.getElementById("studentgrade3");  

if (((parseInt(stud1.value) == 78)) && ((parseInt(stud2.value) == 49)) && ((parseInt(stud3.value) == 90)))
{
alert("correct");
}
else
{
alert("error correct those values");
}

}