为什么单击时总是弹出“ you rock”警报？

Question

I am a new javascript learner. I did this but don't understand why always "you rock" alert pops up when I click. Thanks in advance

 function favColors() { var example = document.getElementById('example'); if(document.getElementById('favorite').value=example.options[0].text) { alert("you rock"); }else { alert("have no color taste"); } document.getElementById('favorite').value=example.options[example.selectedIndex].text; } 

 <form> <select id="example" onChange="favColors()"> <option>Black</option> <option>Red</option> <option>White</option> <option>Pink</option> </select> <p> Your favorite sport is: <input type="text" id="favorite"> </p> </form> 

Answer 1

Your IF clause needs a == instead of a single = . It should look like this:

if(document.getElementById('favorite').value==example.options[0].text) {

instead of

if(document.getElementById('favorite').value=example.options[0].text) { .

Hope it helps.

Also, this:

document.getElementById('favorite').value=example.options[example.selectedIndex].text;

needs to be the first statement of right after defining example .

Update:

• The example.options[example.selectedIndex].text needs to be applied to document.getElementById('favorite').value before you can use it for comparison.
• The = sign is for assignment and == is for comparison. So your IF statement needed updating accordingly. Read this .

Answer 2

You're using an assignment instead of a comparison operator inside of your if statement.

Assignment operator

//sets x to 5
x = 5
//sets y to an empty object
y = {}
//sets y to 5
y = x

Regular comparison operator

//evaluates to true
5 == 5
//evaluates to true
'5' == '5'
//evaluates to true
'5' == 5
//evaluates to false
'5' == 0

Strict comparison operator (the one you should be using)

//evaluates to true
5 === 5
//evaluates to true
'5' === '5'
//evaluates to false
'5' === 5