此JavaScript代码有什么问题？

Question

var input = document.getElementById("myInput");
a.onkeydown = function() {
    if (a.value.match("hi")) {
        a.value.replace("hi", "span style='color:red;'>hi</span>");
    }
};

<input type="text" id="myInput" />

It does not take any action to replace 'hi' with a red 'hi'.

Answer 1

除非您省略某些内容，否则变量a不会代表您的文本框。

Answer 2

You're missing the opening < before the span, for one. Also, I don't know if you can nest HTML inside an input element. A better way would be to turn on a class/style that makes the text red if the value == 'hi'.

Answer 3

1. You are using variable a which is not declared anywhere, I assume you mean input
2. A better option would be to use onkeyup which will trigger the event as soon as you type hi
3. After you do the replace you need to assign it to something.
4. Make sure the <script/> is after the element so the element is ready in the dom.

Putting that all together you get this:

<input type="text" id="myInput" />
<script type="text/javascript">
    var input = document.getElementById("myInput");

    input.onkeyup = function() {
      if (input.value.match("hi")) {
      input.value = input.value.replace("hi", "<span style='color:red;'>hi</span>");
    }
};
</script>

Code example on jsfiddle .