Explain var _t12 = _t12 || [];

I came across this question in a past exam paper and need someone to explain what does the following java script code do

var _t12 = _t12 || [];

var _t12 is a variable declaration. In my idea, this script creates an array with no array size (array size to be put inside the square brackets). But I’m not sure about the modulus operator (||);

In java (not javascript), someArray = new int [7]; declares and creates an array to hold 7 array elements.

Can someone tell me if i’m right or wrong and if wrong what does it do

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