How can I trigger the zoom event, once I've set scale manually on the zoom behavior?

var zoom = d3.behavior.zoom()
    .scaleExtent([0.5, 4])
    .on('zoom', onzoom);

// later on
zoom.scale(2);

https://github.com/mbostock/d3/wiki/Zoom-Behavior#wiki-scale

javascript d3.js
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