I reorganized the node object as an object containing objects. I used particular format because property names are strings, they not sequential integers.

homework.js

/*
    A  ----  B  ----- C
    |        |
    |        |
    D ------ E ------ F
*/

var node = { 
        "A" : {"id": 1, "value": 100, "neighbours": ["D","B"]},
	"B" : {"id": 2, "value": 20, "neighbours": ["A","E","C"]}, 
	"C" : {"id": 3, "value": 30, "neighbours": ["B"]},
	"D" : {"id": 4, "value": 40, "neighbours": ["A","E"]},
	"E" : {"id": 5, "value": 50, "neighbours": ["D","B","F"]}
	};

var max = 0; 

for( var key in node){
	if( max < node[key]["value"] ){
		max = node[key]["value"];
	}
}

document.getElementById('text').innerHTML = "Maximum is : "+max;

index.html

<div id="text"></div>