怎样让FileReader返回base64？

Question

传入一个blob然后打印出来提示undefined 哪里写的不对呢？

function blobToDataURL (blob) {
    let base64
    var a = new FileReader()
    a.readAsDataURL(blob)
    a.onload = function (e) {
      base64 = e.target.result
      return base64
    }
}

Answer 1

方法是异步的当然打印undefined,可以用promise包装下

function blobToDataURL (blob) {
    return new Promise((resolve, reject) => {
        let base64
        var a = new FileReader()
        a.readAsDataURL(blob)
        a.onload = function (e) {
          base64 = e.target.result
          resolve(base64)
        }
    }) 
}
blobToDataURL(blob).then(res => console.log(res))