Javascript evaluating undefined expressions in OR statement -


so have statement looks this

subsetsumarray[i][j]=(subsetsumarray[i-1][j] || subsetsumarray[i-1][j-arr[i]]);

the problem when variable j less arr[i], subsetsumarray[i-1][j-arr[i]] return undefined. if subsetsumarray undefined treated false. whats shortest way this. i'd want work
(some undefined variable || true) ===> return true

thanks in advance help!

the shortest way double negative (!!) trick: 

subsetsumarray[i][j]=!!(subsetsumarray[i-1][j] || subsetsumarray[i-1][j-arr[i]]);

see below code , run it:

// empty array  var = [];  // var[1] undefined  alert( a[1] );                // undefined  alert(!! a[1] );              // false  alert(a[1] || true);          // true  alert( a[1] || a[2] );        // undefined  alert( !! (a[1] || a[2]) );   // false


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