Getting undefined error when trying to select an image using javascript

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I am trying to select an image using an input tag from JavaScript, however I am getting undefined error. Please see my code below, I am getting error at input.files[0]

How can I select an image?

function loadImage(input)
{
var file = input.files[0];
var FileReader = new FileReader();
...

}

What I have tried:

Tried googling with no luck, usually input.files work fine