js大神们，帮忙看下代码，为什么执行不了呢

    <script type="text/javascript">
    function fangfa(){
    var str1=document.getElementById("num1").value;
    var str2=document.getElementById("num2").value;
        if(document.getElementById("number0").innerHTML=="+"){  
    var str3=Number(str1)+Number(str2);
    document.getElementById("zhi").innerHTML=str3;
    }
        if(document.getElementById("number1").innerHTML=="-"){
    var str3=Number(str1)-Number(str2);
    document.getElementById("zhi").innerHTML=str3;
    }
        if(document.getElementById("number2").innerHTML=="*"){
    var str3=Number(str1)*Number(str2);
    document.getElementById("zhi").innerHTML=str3;
    }
        if(document.getElementById("number3").innerHTML=="/"){
    var str3=Number(str1)/Number(str2);
    document.getElementById("zhi").innerHTML=str3;
    }
        if(document.getElementById("number4").innerHTML=="%"){
    var str3=Number(str1)%Number(str2);
    document.getElementById("zhi").innerHTML=str3;
    }}
    </script>       
    <table border="1" >
        <tr>
            <td>输入第一个数：</td>
            <td colspan="4"><input id="num1" type="text"/></td>
        <tr>
            <td>输入第二个数：</td>
            <td colspan="4"><input id="num2" type="text"/></td>
        <tr>
            <td><button onclick="fangfa()" id="number0">+</button></td>
            <td><button onclick="fangfa()" id="number1">-</button></td>
            <td><button onclick="fangfa()" id="number2">*</button></td>
            <td><button onclick="fangfa()" id="number3">/</button></td>
            <td><button onclick="fangfa()" id="number4">%</button></td>
        <tr>
            <td colspan="5" id="zhi"></td>  
    </table>

你的if语句全部匹配了，会执行最后的%操作替换之前的计算结果了

 <script type="text/javascript">
    function fangfa(btn) {
        var str1 = Number(document.getElementById("num1").value);
        var str2 = Number(document.getElementById("num2").value);
        var str3;
        switch (btn.innerHTML) {
            case "+": str3 = str1 + str2; break;
            case "-": str3 = str1 - str2; break;
            case "*": str3 = str1* str2; break;
            case "/": str3 = str1 / str2; break;
            case "%": str3 = str1 % str2; break;
        }
        document.getElementById("zhi").innerHTML = str3;
    }
</script>
<table border="1">
    <tr>
        <td>输入第一个数：</td>
        <td colspan="4"><input id="num1" type="text" /></td>
    <tr>
        <td>输入第二个数：</td>
        <td colspan="4"><input id="num2" type="text" /></td>
    <tr>
        <td><button onclick="fangfa(this)" id="number0">+</button></td>
        <td><button onclick="fangfa(this)" id="number1">-</button></td>
        <td><button onclick="fangfa(this)" id="number2">*</button></td>
        <td><button onclick="fangfa(this)" id="number3">/</button></td>
        <td><button onclick="fangfa(this)" id="number4">%</button></td>
    <tr>
        <td colspan="5" id="zhi"></td>
</table>

    <script type="text/javascript">
    function fangfa(op){
        var str1=document.getElementById("num1").value;
        var str2=document.getElementById("num2").value;
        var str3="";
        if(op=="+") str3=Number(str1)+Number(str2);
        if(op=="-") str3=Number(str1)-Number(str2);
        if(op=="*") str3=Number(str1)*Number(str2);
        if(op=="/") str3=Number(str1)/Number(str2);
        if(op=="%") str3=Number(str1)%Number(str2);
        document.getElementById("zhi").innerHTML=str3;
    }
    </script>
    <table border="1" >
        <tr>
            <td>输入第一个数：</td>
            <td colspan="4"><input id="num1" type="text"/></td>
        <tr>
            <td>输入第二个数：</td>
            <td colspan="4"><input id="num2" type="text"/></td>
        <tr>
            <td><button onclick="fangfa('+')" id="number0">+</button></td>
            <td><button onclick="fangfa('-')" id="number1">-</button></td>
            <td><button onclick="fangfa('*')" id="number2">*</button></td>
            <td><button onclick="fangfa('/')" id="number3">/</button></td>
            <td><button onclick="fangfa('%')" id="number4">%</button></td>
        <tr>
            <td colspan="5" id="zhi"></td>
    </table>