要設置HTML輸入框的默認值，主要使用value屬性。具體方法如下：1. 在<input>標籤中添加value屬性，如，頁面加載時會顯示默認值；2. 區(qū)分value與placeholder，前者為實際默認值，後者僅為提示文本；3. 可通過JavaScript動態(tài)修改默認值，例如使用document.querySelector("input[name='username']").value = "NewDefault";實現(xiàn)交互邏輯中的更新操作。

Sure, here's how to set a default value for an HTML input field — it's pretty straightforward and commonly used when you want to pre-fill forms or provide placeholder suggestions.

Use the value Attribute

The most direct way is using the value attribute inside your <input> tag. Whatever you put in there will show up as the initial content of the input box.

For example:

 <input type="text" name="username" value="JohnDoe">

This shows "JohnDoe" already filled in the input field when the page loads.

You can use this with different input types like numbers, emails, or even hidden fields.

Consider Using placeholder (But It's Not the Same)

A lot of people confuse placeholder with a default value, but they're not the same thing. The placeholder text disappears as soon as the user starts typing, while the value stays until changed.

Use placeholder if you're giving a hint rather than setting a real default:

 <input type="text" name="email" placeholder="example@example.com">

So keep in mind:

• Use value for actual defaults
• Use placeholder only for temporary hints

Resetting or Updating Default Values with JavaScript

If you need to dynamically change the default value based on user actions or other conditions, you can do that with JavaScript.

Here's a quick example:

 document.querySelector("input[name='username']").value = "NewDefault";

This might be useful if you have form steps or conditional logic where inputs should change based on selections.

Just remember: once you set a value with JavaScript, it behaves just like a regular default — unless the user changes it, that value will stay.

Wrapping Up

Setting a default value in an HTML input field mostly comes down to the value attribute. You can enhance or modify it later with JavaScript if needed, and don't mix it up with placeholder .

That's basically it — simple, but useful in a lot of forms.

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