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 var mainarray=[[],[1],[1,2],[1,2,3],[1,2,3,4,5]]; // your array containing arrays
var left=mainarray.map(el=>typeof el=="object"?el.length:0).sort().reduce((num,length)=>num+1==length?num+1:num,-1)+1;
if(left==mainarray.length){
left=0;
}
alert(left);
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