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The FileReader.onload property contains a event handler executed when the load event is fired, when content read with readAsArrayBuffer, readAsBinaryString, readAsDataURL or readAsText is available.
Example
// Callback from a <input type="file" onchange="onChange(event)">
function onChange(event) {
var file = event.target.files[0];
var reader = new FileReader();
reader.onload = function(event) {
// The file's text will be printed here
console.log(event.target.result)
};
reader.readAsText(file);
}