correct way, when changing object, firstly you should create this object and then assign its property to the new object
<!DOCTYPE html>
<html lang="en">
<head>
<title>Chapter 11: Example 7</title>
</head>
<body>
<form action="" name="theForm">
<select name="theDay" size="5">
<option value="0" selected="selected">Monday</option>
<option value="1">Tuesday</option>
<option value="2">Wednesday</option>
<option value="3">Thursday</option>
<option value="4">Friday</option>
<option value="5">Saturday</option>
<option value="6">Sunday</option>
</select>
<br />
<input type="button" value="Remove Wednesday" name="btnRemoveWed" />
<input type="button" value="Add Wednesday" name="btnAddWed" />
<br />
</form>
<script>
var theForm = document.theForm;
function btnRemoveWedClick() {
var options = theForm.theDay.options;
if (options[2].text == "Wednesday") {
options[2] = null;
} else {
alert("There is no Wednesday here!");
}
}
//change object value
function btnAddWedClick() {
var options = theForm.theDay.options;
console.log('length ' +options.length); //6
if (options[2].text != "Wednesday") {
var lastOption = new Option();
options[options.length] = lastOption;
for (var index = options.length - 1; index > 2; index--) {
console.log(index); // 6 because you add last option to options
var currentOption = options[index];
var previousOption = options[index - 1];
currentOption.text = previousOption.text;
currentOption.value = previousOption.value;
}
var option = new Option("Wednesday", 2);
options[2] = option;
} else {
alert("Do you want to have TWO Wednesdays?");
}
}
theForm.btnRemoveWed.addEventListener("click", btnRemoveWedClick);
theForm.btnAddWed.addEventListener("click", btnAddWedClick);
</script>
</body>
</html>
wrong way. don't assign one object to another one , this won't work
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Test</title>
</head>
<body>
<form action="" name="theForm">
<select name="theDay" size="5">
<option value="0" selected="selected">Monday</option>
<option value="1">Tuesday</option>
<option value="2">Wednesday</option>
<option value="3">Thursday</option>
<option value="4">Friday</option>
<option value="5">Saturday</option>
<option value="6">Sunday</option>
</select>
<br />
<input type="button" value="Remove Wednesday" name="btnRemoveWed" />
<input type="button" value="Add Wednesday" name="btnAddWed" />
<br />
</form>
<script>
var theDayEls = document.theForm.theDay;
document.write('option length is :');
document.write(theDayEls.length + ', ' + theDayEls.options.length +'<br>');
console.log(document.theForm.theDay.options)
for (let i=0; i<theDayEls.options.length; i++){
document.write(' | value :');
document.write(theDayEls.options[i].value);
document.write(', text :');
document.write(theDayEls.options[i].text);
}
var btnRemoveWed = document.theForm.btnRemoveWed;
var btnAddWed = document.theForm.btnAddWed;
btnRemoveWed.addEventListener('click',btnRemoveWedClick);
btnAddWed.addEventListener('click',btnAddWedClick);
function btnRemoveWedClick(e) {
var options = document.theForm.theDay.options;
//iterate 7 times, not user-friendly
// for (let option of options){
//
// if (option.text == options[2].text){
// console.log('before');
// console.log(options[2]);
// console.log(option);
// // option= null; //for of loop can not change the value of collections
// //copy the options elements to options for reading
// options[2]= null; //really remove
// console.log('after');
// console.log(options[2]); // turn to options[3] value
// console.log(option); //still exists
// }else {
// alert('no more to be removed');
// }
// }
if (options[2].text == 'Wednesday'){
options[2] = null;
}else {
alert('no more to be removed');
}
}
//can not assign the option object to another option object
//totally wrong
function btnAddWedClick() {
var options =document.theForm.theDay.options;
// var lastOption = options[options.length]; //wrong way
// console.log(lastOption); //undefined
var lastOpt = new Option();
// options[options.length] = lastOpt;
options[options.length] = options[6];
lastOpt =options[6];
console.log(options[5])
console.log(lastOpt)
var prevOpt;
// for (var i = options.length-1 ; i >= 3 ; i--){
// prevOpt = options[i-1];
// var currOpt = options[i];
// var nextOpt = options[i+1];
// nextOpt = currOpt;
// currOpt = prevOpt;
// // console.log(i-1 +'inside for' +(options[2] == prevOpt)); //true
// }
var newWenOpt = new Option('Wednesday',2);
// console.log(prevOpt);
// prevOpt = newWenOpt;
options[2] = newWenOpt;
console.log('outside for' +(options[2] == prevOpt)); //false, there are not the same one
}
</script>
</body>
</html><!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Test</title>
</head>
<body>
<form action="" name="theForm">
<select name="theDay" size="5">
<option value="0" selected="selected">Monday</option>
<option value="1">Tuesday</option>
<option value="2">Wednesday</option>
<option value="3">Thursday</option>
<option value="4">Friday</option>
<option value="5">Saturday</option>
<option value="6">Sunday</option>
</select>
<br />
<input type="button" value="Remove Wednesday" name="btnRemoveWed" />
<input type="button" value="Add Wednesday" name="btnAddWed" />
<br />
</form>
<script>
var theDayEls = document.theForm.theDay;
document.write('option length is :');
document.write(theDayEls.length + ', ' + theDayEls.options.length +'<br>');
console.log(document.theForm.theDay.options)
for (let i=0; i<theDayEls.options.length; i++){
document.write(' | value :');
document.write(theDayEls.options[i].value);
document.write(', text :');
document.write(theDayEls.options[i].text);
}
var btnRemoveWed = document.theForm.btnRemoveWed;
var btnAddWed = document.theForm.btnAddWed;
btnRemoveWed.addEventListener('click',btnRemoveWedClick);
btnAddWed.addEventListener('click',btnAddWedClick);
function btnRemoveWedClick(e) {
var options = document.theForm.theDay.options;
//iterate 7 times, not user-friendly
// for (let option of options){
//
// if (option.text == options[2].text){
// console.log('before');
// console.log(options[2]);
// console.log(option);
// // option= null; //for of loop can not change the value of collections
// //copy the options elements to options for reading
// options[2]= null; //really remove
// console.log('after');
// console.log(options[2]); // turn to options[3] value
// console.log(option); //still exists
// }else {
// alert('no more to be removed');
// }
// }
if (options[2].text == 'Wednesday'){
options[2] = null;
}else {
alert('no more to be removed');
}
}
//can not assign the option object to another option object
//totally wrong
function btnAddWedClick() {
var options =document.theForm.theDay.options;
// var lastOption = options[options.length]; //wrong way
// console.log(lastOption); //undefined
var lastOpt = new Option();
// options[options.length] = lastOpt;
options[options.length] = options[6];
lastOpt =options[6];
console.log(options[5])
console.log(lastOpt)
var prevOpt;
// for (var i = options.length-1 ; i >= 3 ; i--){
// prevOpt = options[i-1];
// var currOpt = options[i];
// var nextOpt = options[i+1];
// nextOpt = currOpt;
// currOpt = prevOpt;
// // console.log(i-1 +'inside for' +(options[2] == prevOpt)); //true
// }
var newWenOpt = new Option('Wednesday',2);
// console.log(prevOpt);
// prevOpt = newWenOpt;
options[2] = newWenOpt;
console.log('outside for' +(options[2] == prevOpt)); //false, there are not the same one
}
</script>
</body>
</html>
