combination.js

var lines = [
    {id: 'L1'}, {id: 'L2'}
];

var demands = [
    {id: 'D1'}, {id: 'D2'}, {id: 'D3'}
];

var line_demands = [];
var combinations = [];
var tempCombs = [];

for (let indexD = 0; indexD < demands.length; indexD++) {
    const elementD = demands[indexD];

    for (let indexL = 0; indexL < lines.length; indexL++) {
        const elementL = lines[indexL];        
        line_demands.push({id: elementL.id + '_' + elementD.id});
    }    
}

for (let index = 0; index < line_demands.length; index++) {
    console.log(line_demands[index]);    
}

console.log("\n");

comb(0, 3);

function comb(offset, k){
    if(k == 0){
        // todo: remove invalid inputs in advanced
        combinations.push(tempCombs.slice()); // slice to avoid reference
        return;
    }
    else{
        for (let index = offset; index <= line_demands.length - k; index++) {
            tempCombs.push(line_demands[index]);
            comb(index + 1, k - 1);
            tempCombs.pop();
        }
    }
}
// todo: remove global bad inputs???
console.log(combinations);

function getDemand(element){
    return element.substring(3);
}

/*
{ id: 'L1_D1' }
{ id: 'L2_D1' }
{ id: 'L1_D2' }
{ id: 'L2_D2' }

var result = [
    [ { id: 'L1_D1' }, { id: 'L2_D1' } ], // invalido
    [ { id: 'L1_D2' }, { id: 'L2_D2' } ], // invalido

    [ { id: 'L1_D1' }, { id: 'L1_D2' } ], // pessimo
    [ { id: 'L2_D1' }, { id: 'L2_D2' } ], // pessimo

    [ { id: 'L2_D1' }, { id: 'L1_D2' } ], // otimo
    [ { id: 'L1_D1' }, { id: 'L2_D2' } ], // otimo

]


*/