When a d3.behavior.zoom event is triggered programatically, how do you set inital values for translate and scale? - d3.js

Question

The squares in the example below are part of an SVG group that has an initial translate and scale set.
Clicking on a square initiates a zoom transition. But the intial values set by the transition are different from my defaults, as made obvious by the jarring start to this transition.
How can I set initial values for translate and scale on a zoom transition that I initiate programatically?
var svg = d3.select("#main");
svg.append("rect").attr({"x":0,"y":0,"height":100,"width":100,"fill":"red"})
svg.append("rect").attr({"x":100,"y":100,"height":100,"width":100,"fill":"blue"})
svg.append("rect").attr({"x":0,"y":100,"height":100,"width":100,"fill":"green"})
svg.append("rect").attr({"x":100,"y":0,"height":100,"width":100,"fill":"yellow"})
var zoom = d3.behavior.zoom().on("zoom",function(){
var t = d3.event.translate;
var s = d3.event.scale;
console.log(s)
svg.attr("transform","translate("+t[0]+","+t[1]+") scale("+s+")")
}).scaleExtent([1,10]).scale(1).translate([0,0])
d3.select("svg").call(zoom)
d3.selectAll("rect").on("mousedown",function(){
var scale = Math.random()*3;
var translate = [Math.random()*200,Math.random()*200]
zoom.scale(scale);
zoom.translate(translate);
//new transition
var T = svg.transition().duration(5000)
zoom.event(T);
})
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js"></script>
<p style="font-weight:bold">When a zoom is triggered programatically, how do you set inital values for translate and scale?</p>
<p>Click on one of the squares</p>
<svg height="600px" width="600px">
<g id="main" transform="translate(25,25) scale(0.25)"></g>
</svg>

Answer 1

That is a problem with the zoom function itself. I would suggest zooming the children as opposed to the parent if that would work
var zoom = d3.behavior.zoom().on("zoom",function(){
var t = d3.event.translate;
var s = d3.event.scale;
svg.selectAll("rect").attr("transform","translate("+t[0]+","+t[1]+") scale("+s+")")
}).scaleExtent([1,10]);
EDIT
The problem with the above code is that d3.js does not register the transformation or initial state of the SVG. This problem runs deeper. As d3 does not keep track of the SVG transformations and just executes them. It only keeps track of transformations you've run on the library in a variable called __chart__.
So when the zoom function is run it just interpolates the variables and gives the output. As no functions have been run on this yet the __chart__ variable has not been set and causing the jerky start from (x=0, y=0, k=1).
Solution:
Run this code before the zoom transformation to set the initial chart manually
svg.transition().each(function(){
this.__chart__={x:25,y:25,k:0.25}; //or you can pick those values using attr
});
Zoom the svg programmatically to 25,25,0.25 first before any other function. (this is why your workaround works as the __chart__ variable gets set)

Answer 2

To set the initial value of the zoom, try something like this:
// Init zoom
var zoom = d3.behavior.zoom().on("zoom", function () {
svg.attr("transform", "translate(" + d3.event.translate + ")" + " scale(" + d3.event.scale + ")");
});
// Get SVG element
var svg = d3.select("svg")
.call(zoom)
.append("g");
// Create circle
svg.append("circle")
.attr("cx",0)
.attr("cy",0)
.attr("r", 5);
// Create init value
var scale = 5;
var translate = [50, 50];
// Set init value
zoom.scale(scale);
zoom.translate(translate);
// Call zoom event
svg.call(zoom.event);
// or svg.transition().call(zoom.event);
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js"></script>
<svg height="100px" width="100px"></svg>

Answer 3

I was looking for the answer to this, but it seems D3 has already evolved a couple of versions.
Although Majkl and cjds's answers helped me solve my problem, I thought it would help to leave more up to date information, since it was hard finding v5.4 examples out there, until I found Observable at least.
// Applies event transformation to the Group element's attribute
const zoom_action = () => g.attr("transform", d3.event.transform)
// Create the zoom handler
const zoom = d3
.zoom()
.on("zoom", zoom_action)
// Get SVG element and apply zoom behaviour
var svg = d3
.select("svg")
.call(zoom)
// Create Group that will be zoomed
var g = svg.append("g")
// Create circle
g.append("circle")
.attr("cx",0)
.attr("cy",0)
.attr("r", 5)
// Set initial scale and translation
zoom.scaleBy(svg, 5)
zoom.translateBy(svg, 50, 50)
<script src="https://d3js.org/d3.v5.js"></script>
<svg height="100px" width="100px"></svg>