[javascript] How to go from Blob to ArrayBuffer

Just to complement Mr @potatosalad answer.

You don't actually need to access the function scope to get the result on the onload callback, you can freely do the following on the event parameter:

var arrayBuffer;
var fileReader = new FileReader();
fileReader.onload = function(event) {
    arrayBuffer = event.target.result;
};
fileReader.readAsArrayBuffer(blob);

Why this is better? Because then we may use arrow function without losing the context

var fileReader = new FileReader();
fileReader.onload = (event) => {
    this.externalScopeVariable = event.target.result;
};
fileReader.readAsArrayBuffer(blob);