Javascript Problem with the array.map function

Question

Here I have a strange problem, I have a data table of the form:

[{grade:"a", count:1000},
{grade:"b", count:935},
.....]

but when I use the map function :

var v = data.map(function (d) { return d.count; });
console.log(v);

table v is found with the values {0,0,0,0,0} If anyone has an idea where the problem may come from, I'm interested.

var svg = d3.select("svg"),
    margin = { top: 20, right: 20, bottom: 30, left: 40 },
    x = d3.scaleBand().padding(0.1),
    y = d3.scaleLinear();

var g = svg.append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

g.append("g")
    .attr("class", "axis axis--x");

g.append("g")
    .attr("class", "axis axis--y");

g.append("text")
    .attr("transform", "rotate(-90)")
    .attr("y", 6)
    .attr("dy", "0.71em")
    .attr("text-anchor", "end")
/*.text("Count of Product")*/;

var dm = data_manager();
var data = dm.load_histo_grade_data();

console.log(data);

x.domain(data.map(function (d) { return d.grade; }));
var v = data.map(function (d) { return d.count; });
console.log(v);
y.domain([0, d3.max(data.map(function (d) { return d.count; }))]);

draw(data);

function draw(theData) {

    var margin = {top: 10, right: 30, bottom: 30, left: 40},
	width = 300 - margin.left - margin.right,
	height = 200 - margin.top - margin.bottom;

    x.rangeRound([0, width]);
    y.rangeRound([height, 0]);

    g.select(".axis--x")
	.attr("transform", "translate(0," + height + ")")
	.call(d3.axisBottom(x));

    
    g.select(".axis--y")
	.call(d3.axisLeft(y));

    // ENTER
    g.selectAll(".bar")
	.data(theData)
	.enter().append("rect")
	.attr("class", "bar")
	.attr("x", function (d) { return x(d.grade); })
	.attr("y", function (d) {
	    return y(d.count); })
	.attr("width", x.bandwidth())
	.attr("height", function (d) { return height - y(d.count); })
	.attr("fill", function(d) { return color_nutriscore_grade(d.grade); } );
}

Edit : The code for the function data_manager and load_histo_grade_data

function data_manager(){
    var dm = {};
    dm.filter = {
	WORLD:0,
	REGION:1
    };
    dm.choosen_filter = dm.filter.WORLD;
    dm.choosen_region = "Europe";
    dm.accept = function(row){
	return true;
    };
    dm.load_histo_grade_data = function(){
	data_to_update = [
	    {grade:"a", count:0.0},
	    {grade:"b", count:0.0},
	    {grade:"c", count:0.0},
	    {grade:"d", count:0.0},
	    {grade:"e", count:0.0}
	];
	d3.tsv("../tsv/hypotesis.tsv", function(error, data){
	    if(error) throw error;
	    var tmp = {
		"a":0,
		"b":1,
		"c":2,
		"d":3,
		"e":4,
	    };
	    for(var i = 1; i<data.length; i++){
		idx = tmp[data[i].grade];
		var v = data_to_update[idx].count;
		data_to_update[idx].count = (v + 1);
	    }
	});
	return data_to_update;
    };
    dm.load = function(){
	dm.load_histo_grade_data();
    }
    return dm;
}

function color_nutriscore_grade(grade){
    switch(grade) {
    case '':  return d3.rgb(191, 191, 191);
    case 'a': return d3.rgb(  0, 191,   0);
    case 'b': return d3.rgb(115, 255,   0);
    case 'c': return d3.rgb(255, 204,   0);
    case 'd': return d3.rgb(255, 102,   0);
    case 'e': return d3.rgb(255,  25,   0);
    };
}

The problem is likely caused by the array not looking like you think it looks. — Pointy
– Pointy, Commented Dec 26, 2018 at 19:08
The output for the consle log is : (5) […] 0: Object { grade: "a", count: 1009 } 1: Object { grade: "b", count: 935 } 2: Object { grade: "c", count: 1514 } 3: Object { grade: "d", count: 2198 } 4: Object { grade: "e", count: 1690 } length: 5 <prototype>: [] — litsubzu
– litsubzu, Commented Dec 26, 2018 at 19:15
d3.tsv looks like it is asynchronous, that's where your problem is. — Herohtar
– Herohtar, Commented Dec 26, 2018 at 20:01

robinsax · Accepted Answer · 2018-12-26 20:08:38Z

The problem here is that d3.tsv() is asynchronous, meaning it will run your callback "later". So you're returning the initial value of data_to_update (with all grades 0) from dm.load_histo_grade_data(), then later it is being populated with the correct values.

The reason your call to console.log(data) is outputting the populated array is because console.log has the fancy feature of displaying a live object, meaning it the object changes after it was logged, what you see in the console will change too.

The fix is to pass a callback into dm.load_histo_grade_data() instead of waiting for it to return, like so:

// In data_manager()
dm.load_histo_grade_data = function(callback){
    data_to_populate = [
        {grade:"a", count:0.0},
        {grade:"b", count:0.0},
        {grade:"c", count:0.0},
        {grade:"d", count:0.0},
        {grade:"e", count:0.0}
    ];
    d3.tsv("../tsv/hypotesis.tsv", function(error, data){
        if(error) throw error;
        var tmp = {
            "a":0,
            "b":1,
            "c":2,
            "d":3,
            "e":4,
        };
        for (var i = 1; i<data.length; i++){
            idx = tmp[data[i].grade];
            var v = data_to_populate[idx].count;
            data_to_populate[idx].count = (v + 1);
        }
        callback(data_to_populate)
    });
};

You can then use this as follows:

var dm = data_manager();
dm.load_histo_grade_data(function(data) {
    console.log(data);

    x.domain(data.map(function (d) { return d.grade; }));
    var v = data.map(function (d) { return d.count; });
    console.log(v);
    y.domain([0, d3.max(data.map(function (d) { return d.count; }))]);

    draw(data);
});

Side note: You should use camelCase instead of snake_case in JavaScript.

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