Why filereader.onload doesn't execute the first time?

Question

var k=-1
var j=-1
var data={}
var data_h={}
var finish=false

chrome.webRequest.onBeforeSendHeaders.addListener(
    function(details) {
        j++;
        console.log('BeforeSendHeaders',j)
        console.log(finish)
        if(finish==false){
        //{ console.log('head')
        an=details.requestHeaders[1].value


        data_h[j]=an
        console.log(data_h)
        }   
    }
    , {
        urls: ["https://mms.pinduoduo.com/sydney/api/goodsDataShow/queryGoodsSpanDateList"]
    },
    ["blocking", "requestHeaders", "extraHeaders"]

)

 chrome.webRequest.onBeforeRequest.addListener(


    function(details)
    {
        k++;
        console.log('beforerequest',k)

        if(finish==false)
        {

        buffer=details.requestBody.raw[0].bytes     
        console.log(buffer)  
        var blob = new Blob([buffer]);
//将 Blob对象 读成字符串
        var reader = new FileReader();
        reader.onloadend = function (e) {
        setTimeout(function(){ var send=reader.result; //a Hello world!
                console.log(k,"次")

        data[k]=send}, 1);          
        }
        reader.readAsText(blob, 'utf-8');           
        }

        //} 
    },
    {urls: ["https://mms.pinduoduo.com/sydney/api/goodsDataShow/queryGoodsSpanDateList"]},
    ['requestBody']
); 

chrome.webRequest.onBeforeSendHeaders.addListener(
    function(details) {



        for (var j = 0; j < details.requestHeaders.length; ++j) {
            if (details.requestHeaders[j].name === 'Origin')
                details.requestHeaders[j].value = 'https://mms.pinduoduo.com';
        }



    }, {
        urls: ["https://mms.pinduoduo.com/sydney/api/goodsDataShow/queryGoodsSpanDateList"]
    },
    ["blocking", "requestHeaders", "extraHeaders"]

)

 chrome.runtime.onMessage.addListener(function(request, sender, sendResponse)
{
    finish=request

    sendResponse({head:data_h,body:data});
});

/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////// This code gets the requestbody and the request head and send them to content.js. The console prints 'k' start from 1,but not 0.I don't know why the first time filereader.onload doesn't execute the first time. I tried that console.log(buffer) can print correctly,but console.log(k,"次") is not.I guess that it's only because of reader.onload