请求 blob 图像并使用 fetch API 转换为 base64

通用功能：

function urlContentToDataUri(url){
    return  fetch(url)
            .then( response => response.blob() )
            .then( blob => new Promise( callback =>{
                let reader = new FileReader() ;
                reader.onload = function(){ callback(this.result) } ;
                reader.readAsDataURL(blob) ;
            }) ) ;
}

//Usage example:
urlContentToDataUri('http://example.com').then( dataUri => console.log(dataUri) ) ;

//Usage example using await:
let dataUri = await urlContentToDataUri('http://example.com') ;
console.log(dataUri) ;