JS正则函数里面放变量

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我要用 JS的正则 替换字符串里面的关键字（变量）

错误示例

    var str = '2';
    var a = '1232'
    a = a.replace(/str/g, "b");
    //1232

正确示例

var str ='2';
var a ='1232'
a=a.replace(new RegExp(str,'g'),"b");
//1b3b

需求2，有一个数组，里面都是关键字，想判断数组里面的某个下标的值是否在一个字符串中存在  示例代码

let arr1= ['?', '?', '?', '?', '?', '?', '?', '?', '?', '?', '?',
      '?', '?', '?', '?', '?', '?', '?', '?', '?', '?', '?', '?', '?',
      '?', '?', '?', '?', '?', '?', '?', '?', '?', '?'
    ];
let arr2= ['emoji1', 'emoji2', 'emoji3', 'emoji4', 'emoji5', 'emoji6', 'emoji7', 'emoji8', 'emoji9', 'emoji10', 'emoji11','emoji12', 'emoji13', 'emoji14', 'emoji15', 'emoji16', 'emoji17', 'emoji18', 'emoji19', 'emoji20', 'emoji21', 'emoji22', 'emoji23', 'emoji24','emoji25', 'emoji26', 'emoji27', 'emoji28', 'emoji29', 'emoji30', 'emoji31', 'emoji32', 'emoji33', 'emoji34'];

    let inputValue ='aaa?aa?'
    for (let i = 0; i < arr1.length; i++) {
      inputValue =inputValue.replace(new RegExp(arr1[i],'g'), arr2[i])
    }
    console.log(inputValue) //aaaemoji1aaemoji2