js 这个为啥不判断 直接跳转了页面

Question

Answer 1

这里判断逻辑有问题，filename!=null的意思是只要filename不为null的情况下；而如果未选择未见filename也是不为null，只是为空，因此要判断它的布尔值即可，if(filename){...}；这样当未选择时，alert便会执行；另外，防止跳转首先在函数内写上return false;然后在<input>标签上这么写 onclick="return photo()"；如：<input type="submit" onclick="return photo()"/> function photo() {     if (1 > 0) {         return false;     } }题外话：像这种判断要放后端去做的，前端做被人随便改

Answer 2

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Answer 3

停止的话需要写“return”

Answer 4

你好，要return false; 不然还是会执行下去，或者是判断的不对，可能是空字符串，再判断下空字符串，可以弹出filename看看值。可追问，望采纳！