ต้องการเขียน check email จาก form คือเช็คค่าว่างได้แต่อยากให้มันเช็คค่าว่าเป็น email ด้วยอ่าครับ

<form action="#" onsubmit="return false;">

<div>Enter An Email Address</div>
<div><input type="text" name="address" size="40" value="" >&nbsp;
<button onclick="checkValid(this.form.address);">Test</button>
</div>

</form>

function checkValidation(formInput) {

    if (typeof(formInput) != "object") {
        alert("Validation not supported on this browser.");
        return(false);
    }

    var message;

    if (stringEmpty(formInput.value)) {
        message = "Error! There is no input value entered.";
        alert(message);
    } else if (noAtSign( formInput.value )) {
        message = "Error! The address \"" + formInput.value + "\" does not contain an '@' character.";
        alert(message);
    } else if (nothingBeforeAt(formInput.value)) {
        message = "Error! The address \"" + formInput.value;
        message += "\" must contain at least one character before the '@' character";
        alert(message);
    } else if (noLeftBracket(formInput.value)) {
        message = "Error! The address \"" + formInput.value;
        message += "\" contains a right square bracket ']',\nbut no corresponding left square bracket '['.";
        alert(message);
    } else if (noRightBracket(formInput.value)) {
        message = "Error! The address \"" + formInput.value;
        message += "\" contains a left square bracket '[',\nbut no corresponding right square bracket ']'.";
        alert( message);
    } else if (noValidPeriod(formInput.value)) {
        message = "Error! The address \"" + formInput.value + "\" must contain a period ('.') character.";
        alert(message);
    } else if (noValidSuffix(formInput.value)) {
        message = "Error! The address \"" + formInput.value;
        message += "\" must contain a two, three or four character suffix.";
        alert(message);
    } else {
        message = "Success! The email address \"" + formInput.value + "\" validates OK.";
        alert(message);
    }

    var objType = typeof(formInput.focus);
    if (objType == "object" || objType == "function") {
         formInput.focus();
    }

    return (false);
}

function checkValid (formField) {
    if ( checkValidation ( formField ) == true ) {
        alert ( 'E-Mail Address Validates OK' );
    }

    return ( false );
}

function stringEmpty (formField) {
    // CHECK THAT THE STRING IS NOT EMPTY
    if ( formField.length < 1 ) {
        return ( true );
    } else {
        return ( false );
    }
}

function noAtSign (formField) {
    // CHECK THAT THERE IS AN '@' CHARACTER IN THE STRING
    if (formField.indexOf ('@', 0) == -1) {
        return ( true )
    } else {
        return ( false );
    }
}

function nothingBeforeAt (formField) {
    // CHECK THERE IS AT LEAST ONE CHARACTER BEFORE THE '@' CHARACTER
    if ( formField.indexOf ( '@', 0 ) < 1 ) {
        return ( true )
    } else {
        return ( false );
    }
}

function noLeftBracket (formField) {
    // IF EMAIL ADDRESS IN FORM 'user@[255,255,255,0]', THEN CHECK FOR LEFT BRACKET
    if ( formField.indexOf ( '[', 0 ) == -1 && formField.charAt (formField.length - 1) == ']') {
        return ( true )
    } else {
        return ( false );
    }
}

function noRightBracket (formField) {
    // IF EMAIL ADDRESS IN FORM 'user@[255,255,255,0]', THEN CHECK FOR RIGHT BRACKET
    if (formField.indexOf ( '[', 0 ) > -1 && formField.charAt (formField.length - 1) != ']') {
        return ( true );
    } else {
        return ( false );
    }
}

function noValidPeriod (formField) {
    // IF EMAIL ADDRESS IN FORM 'user@[255,255,255,0]', THEN WE ARE NOT INTERESTED
    if (formField.indexOf ( '@', 0 ) > 1 && formField.charAt (formField.length - 1 ) == ']')
        return ( false );

    // CHECK THAT THERE IS AT LEAST ONE PERIOD IN THE STRING
    if (formField.indexOf ( '.', 0 ) == -1)
        return ( true );

    return ( false );
}

function noValidSuffix(formField) {
    // IF EMAIL ADDRESS IN FORM 'user@[255,255,255,0]', THEN WE ARE NOT INTERESTED
    if (formField.indexOf('@', 0) > 1 && formField.charAt(formField.length - 1) == ']') {
        return ( false );
    }

    // CHECK THAT THERE IS A TWO OR THREE CHARACTER SUFFIX AFTER THE LAST PERIOD
    var len = formField.length;
    var pos = formField.lastIndexOf ( '.', len - 1 ) + 1;
    if ( ( len - pos ) < 2 || ( len - pos ) > 4 ) {
        return ( true );
    } else {
        return ( false );
    }
}