Question

<script>
    window.BlobBuilder = window.BlobBuilder || window.WebKitBlobBuilder;

    Blob.prototype.slice = Blob.prototype.slice || function(start, length) {
        return this.webkitSlice(start, start + length);
    }

</script>

These codes are from a background page of a Chrome extension. Any idea or hint will be highly appreciated.

Answer 1

window.BlobBuilder = window.BlobBuilder || window.WebKitBlobBuilder;

this says set window.BlobBuilder to what it is already set to if whatever it is set to is truthy; if whatever it’s set to currently is falsey, then set it to window.WebKitBlobBuilder. This is a general approach used to assign a value to a variable if it doesn’t already have a truthy value. Truthy values are anything that are not falsey values, and the falsey values are

• null
• undefined
• false
• ''
• 0
• NaN

The next line uses similar syntax

Blob.prototype.slice = Blob.prototype.slice || function(start, length) {
    return this.webkitSlice(start, start + length);
}

This sets the slice function on the Blob prototype object either to itself if the current value is truthy, or to a function that will use the webkitSlice function defined on this (which I assume will be WebKitBlobBuilder in this instance).