SOURCE

let saveObj = {
    "stepInfo": {
        "companyChannel": [
            "1",
            "2",
            "3"
        ],
        "promotionSteps": [
            {
                "minAmount": 12,
                "maxAmount": 333,
                "mainStep": 0,
                "returnBcoupon": [
                    {
                        "batchId": "121",
                        "number": 2,
                        "replaceFastCarCoupon": "1",
                        "fastCarBatchId": "332",
                        "fastCarNumber": 12
                    },
                    {
                        "batchId": "123",
                        "number": 123,
                        "replaceFastCarCoupon": "1",
                        "fastCarBatchId": "3",
                        "fastCarNumber": 4
                    }
                ]
            },
            {
                "minAmount": 3,
                "maxAmount": 122,
                "mainStep": 0,
                "returnBcoupon": [
                    {
                        "batchId": "11",
                        "number": 2,
                        "replaceFastCarCoupon": "1",
                        "fastCarBatchId": "12",
                        "fastCarNumber": 33
                    },
                    {
                        "batchId": "122",
                        "number": 12
                    }
                ]
            },
            {
                "minAmount": 33,
                "maxAmount": 121221,
                "mainStep": 0
            },
            {
                "minAmount": 12,
                "maxAmount": 14,
                "mainStep": 0,
                "returnBcoupon": [
                    {
                        "batchId": "33",
                        "number": 12,
                        "replaceFastCarCoupon": "1",
                        "fastCarBatchId": "12",
                        "fastCarNumber": 33
                    },
                    {
                        "batchId": "44",
                        "number": 12
                    },
                    {
                        "batchId": "4441",
                        "number": 122
                    }
                ]
            }
        ],
        "stepCrowdTag": "1001271955",
        "time": [
            "2022-07-13T09:32:30.252Z",
            "2022-07-22T09:32:30.252Z"
        ],
        "startTime": "2022-07-13 17:32:30",
        "endTime": "2022-07-22 17:32:30"
    }
}
let saveObjLen = saveObj["stepInfo"]["promotionSteps"].length
let saveArrPro = saveObj["stepInfo"]["promotionSteps"]
let arr = []
if(saveObjLen > 1){
    saveArrPro.some((item,index)=>{
       Object.keys(item).some(ele=>{
           if(ele == "minAmount" || ele == "maxAmount"){
               arr.push(item[ele])
           }
       })
    })
}
console.log(arr)