How to set one value in a two dimensional javascript array -


i trying set single value in 2 dimensional array, not working.

consider following code taken many examples on subject:

// create 3x3 array -1 in every position: let = new array(3).fill(new array(3).fill(-1)) console.log(`a: ${json.stringify(a)}`) a[1][2] = 0 console.log(`a: ${json.stringify(a)}`)

the output follows:

a: [[-1,-1,-1],[-1,-1,-1],[-1,-1,-1]] a: [[-1,-1,0],[-1,-1,0],[-1,-1,0]]

as can see, instead of setting single cell, set 3 positions in array 0, i.e. sets [0][2] , [1][2] , [2][2] = 0. odd.

i tried this:

let = new array(3).fill(new array(3).fill(-1)) console.log(`a: ${json.stringify(a)}`) a[1,2] = 0 console.log(`a: ${json.stringify(a)}`)

which gives stranger result:

a: [[-1,-1,-1],[-1,-1,-1],[-1,-1,-1]] a: [[-1,-1,-1],[-1,-1,-1],0]

am going crazy, or javascript not support setting value in 2 dimensional array?

first question:

that's because fill outer array same subarray! a[0], a[1] , a[2] same array! because in js objects (including arrays) passed around using references! array.fill taking parrameter (in case reference array) , assigns every item in array. it's this:

var sub = new array(3);  sub.fill(-1);    var = [];  a[0] = sub;  a[1] = sub;  a[2] = sub;    a[0][1] = 0;    console.log(sub); // changed too

because here array.fill doing:

var = new array(10);  a.fill(math.random());     console.log(a);   // not 10 different random numbers, same random number assigned whole array

second question:

it has nothing arrays. it's comma operator 1 used in loops (for example), group expressions 1 , returning value of last:

console.log((1, 2)); // 2  console.log((1, 5, "last")); // last  console.log((1, 5, 5 * 11 + 10)); // 65    // parens used distinguish comma operator parameter separator

so a[1, 2] same a[2] because value of 1, 2 2!


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