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);