Question

I get an ” Uncaught TypeError: Cannot read property ‘length’ of undefined ” at line 26. I can’t see the reason this is happening, because the console logs line 24 as an array with a value at index 1, but line 25 logs as undefined.

"use strict"
function findpath(G,si,di){
    //G is an array of nodes (with id, lat, lon)
    var P=[si];
    var C=[0,P];
    var M=[C];
    var O=[];
    var ctr=0;
    var done = false;
    var reached = false;
    var best = undefined;


    while(!done){
        ctr++;
        if( ctr > 100 ){
            alert("Sorry, can't find the destination.");
            return P;
        }

        for (var i=0;i<M.length;++i){
            console.log(P);
            console.log(C);
            console.log(M[i]);
            console.log(M[i[0]]);
            var last = M[i[1]].length;
            var v = M[i[1[last]]];

            //select a random neighbor...
            if( v.N.length === 0 ){
                alert("Wat?");
                return [];
            }
            else if( v.N.length === 1 ){
                break;
            }
            else if( v === di ){
                break;
            }
            else {
                for (var j=0;j<v.N.length;++j){
                    var temp = M[i];
                    O.push(temp[1].push(v.N[j]));
                    var dist = distance(v.lat,v.lon,v.N[j].lat,v.N[j].lon);
                    var temp2 = O.length-1;
                    O[temp2[0]]+=dist;
                    if (v.N[j]===di){
                        reached = true;
                        if (best === undefined){
                            console.log("ASSIGN");
                            best = O[temp2];
                        }
                        else {
                            if (O[temp2[0]]<best[0]) {
                                best = O[temp2];
                            }
                        }
                    }
                }
            }
        }
        M = O;
        var any = false;
        for (var i=0;i<M.length;++i) {
            if (M[i[0]]<best[0]) {
                any = true;
            }
        }
        if (!any) {
            done = true;
        }
    }

    //return the path
    return best[1];
}

function distance(x1,y1,x2,y2){
    return Math.sqrt(Math.pow((x2-x1),2)+Math.pow((y2-y1),2));
}

Output:

Array[1]     findpath.js:22
Array[2]     findpath.js:23
Array[2]
   0: 0
   1: Array[1]
      0: 13
   length: 1
   __proto__: Array[0]
   length: 2
   __proto__: Array[0]     findpath.js:24
undefined findpath.js:25
Uncaught TypeError: Cannot read property 'length' of undefined    findpath.js:26

Answer 1

EDIT – per your comments, it looks like you have an array of arrays, and that, if I understand you correctly, M[i[1]] should be M[i][1]

---

You’re checking

console.log(M[i[0]]);

but then accessing

 var last = M[i[1]].length;

it seems as though M[i[0]] contains a valid array, but M[i[1]] does not.

I would recommend taking a closer look at your M, and i arrays to figure out why M[i[1]] is undefined.

---

Edit, as the comment below says, i appears to be the loop control variable. Did you mean to type simply M[i] ?