如果情况总是如此，但可能是假的 [英] If case is always true, but probably false

问题描述

我有一个简单的表格下拉菜单，我想根据选择值显示不同的内容。我有一个名为 connectiontype 的变量，它带有来自下拉列表的正确值，但if / else语句似乎不起作用 - 我总是以红色结束。有什么想法？

I have a simple form drop-down, and I want to display something different depending on the selection value. I have a variable called connectiontype that comes in with the correct values from the drop down, but the if/else statements don't seem to work - I always end up with red. Any ideas as to why?

Add 
<select name="connection_type" id="connection_type">
  <option value="red">Red</option>
  <option value="green">Green</option>
  <option value="blue">Blue</option>
</select>
connection 

<input type="button" value="Go" onclick="javascript:addDataSource();">

这是javascript，简化。

Here's the javascript, simplified.

function addDataSource() {
    DSN++;

    connectiontype = $("#connection_type").val();

    if (connectiontype = 'red') {
        var html =   'Red';
     } else if (connectiontype = 'green') {
        var html =   'Green';
    } else {
        var html =   'Blue';
    }

    addElement('DSN', 'div', 'DSN-' + DSN, html);
    console.log(DSN);
}   

function addElement(parentId, elementTag, elementId, html) {
    var p = document.getElementById(parentId);
    var newElement = document.createElement(elementTag);
    newElement.setAttribute('id', elementId);
    newElement.innerHTML = html;
    p.appendChild(newElement);
}

推荐答案

您正在使用 = （赋值）而不是 == （比较）。

You are using = (assignment) instead of == (comparison).

if (connectiontype == 'red') {
    ...
} else if (connectiontype == 'green') {
    ...
}

当你有作业时，例如： lhs = rhs 整个表达式返回 rhs 。所以：

When you have an assignment, eg: lhs = rhs the whole expression returns whatever rhs is. So:

if (connectiontype = 'red') { ...

// is equivalent to (as far as the "if" is concerned):

if ('red') { ...  

由于'red'（非空字符串）在JavaScript中是truthy， if 始终为true，您的 html 变量将始终设置为'Red'。

Since 'red' (a non-empty string) is "truthy" in JavaScript, the if is always true and your html variable will always be set to 'Red'.

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