d3_v4_transition_test.js

const circle = d3.select('circle');

function transition(radius, duration) {
  circle.transition()
    .duration(duration)
    .attr('r', radius);
}

describe('#transition', () => {
  beforeEach(() => {
    D3TestUtils.stubAndForceTransitions();
  });
  
  afterEach(() => {
    D3TestUtils.restoreTransitions();
  });
  
  it('correctly transitions circle radius', () => {
    transition(50, 1000);
    assert.strictEqual(circle.attr('r'), '50');
  });
});