Reading file contents on the client-side in javascript in various browsers

In order to read a file chosen by the user, using a file open dialog, you can use the tag. You can find information on it from MSDN. When the file is chosen you can use the FileReader API to read the contents.

function onFileLoad(elementId, event) {
    document.getElementById(elementId).innerText = event.target.result;
}

function onChooseFile(event, onLoadFileHandler) {
    if (typeof window.FileReader !== 'function')
        throw ("The file API isn't supported on this browser.");
    let input = event.target;
    if (!input)
        throw ("The browser does not properly implement the event object");
    if (!input.files)
        throw ("This browser does not support the `files` property of the file input.");
    if (!input.files[0])
        return undefined;
    let file = input.files[0];
    let fr = new FileReader();
    fr.onload = onLoadFileHandler;
    fr.readAsText(file);
}


<input type='file' onchange='onChooseFile(event, onFileLoad.bind(this, "contents"))' />
<p id="contents"></p>

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