Christian Schlensker Christian Schlensker - 1 month ago 5
React JSX Question

How to set the width of a React component during test?

I'm trying to test a slider component.

This slider component can be variable in width. When you click on the "track" of the slider it should change the value and trigger an

callback. The value is a based on where you click on the track. If you click the halfway point when the min value is 100 and the max value is 200, then it should report a value of 150.

The problem I'm running into is that when I render the component using
the component doesn't have any width, so it can't calculate a new value when you click on it.

Here is the component

import React, {PropTypes} from 'react';
import ReactDOM from 'react-dom';
import { noop } from 'lodash';
import style from './style.scss';

export default class Slider extends React.Component {
render() {
return (
style={{left: `${this.calculateLeft()}%`}}>
<div className='track'></div>

handleClick(e) {
let node = ReactDOM.findDOMNode(this);
let {clientX, clientY} = e;
let {offsetLeft, offsetWidth, clientWidth} = node;
let xPercent = (clientX - offsetLeft) / offsetWidth;
console.log(offsetLeft, offsetWidth, clientWidth, xPercent);
this.props.onChange(normalize(xPercent, this.props.min, this.props.max));

calculateLeft() {
let numerator = this.props.value - this.props.min;
let denominator = this.props.max - this.props.min;
return numerator / denominator * 100;

// Proptypes
// ----------------------------------------------------------------------------
Slider.propTypes = {
// Callback for when the value changes.
onChange: PropTypes.func,
// The value for when the slider is at 0%
min: PropTypes.number,
// The value for when the slider is at 100%
max: PropTypes.number,
// The starting value
value: validateValue,

Slider.defaultProps = {
onChange: noop,
min: 0,
max: 100,

// Custom Validation
// ----------------------------------------------------------------------------
function validateValue(props, propName, componentName) {
let value = props[propName];

if (typeof(value) !== 'number') {
return new Error(`value must be a number, got ${typeof(value)}`);

if (value > props.max || value < props.min) {
return new Error(
`value: ${value} must be between max: ${props.max}
and min: ${props.min}`

// Helpers
// ---------------------------------------------------------------------------

function normalize(floatValue, min, max) {
let range = max - min;
let normalizedValue = floatValue * range + min;
// cleverly restrict the value be between the min and max
return [min, normalizedValue, max].sort()[1];

Stylesheet (

.Slider {
position: relative;
display: block;
width: 100px;

.track {
height: 4px;
background: #666;
border-radius: 2px;

.handle {
width: 12px;
height: 12px;
background: #fff;
border-radius: 10px;
position: absolute;
top: 50%;
transform: translate(-50%, -50%);
transition: left 100ms linear;

Here is my test:

import Slider from './Slider';
import React from 'react';
import {
} from 'react-addons-test-utils';

describe('Slider', function() {

describe('click', function() {
it('triggers the onChange callback', function() {
const onChange = sinon.spy();
const component = renderIntoDocument(
style={{width: 100, height: 40}}

const track = findRenderedDOMComponentWithClass(component, 'track');, {clientY: 0, clientX: 10})

Test output

LOG LOG: 0, 0, 0, Infinity
✗ triggers the onChange callback
AssertionError: expected onChange to have been called with arguments 10

at /components/Slider/test.js:99 < webpack:///src/components/Slider/test.js:55:6

Those log statements are from the
function in the component.

The width is zero so the denominator ends up being zero when calculating xPercent, which causes it to be Infinity. This causes it to just use the
value of 200.


How do I make the component have width when rendering it during a test?


I've been fighting the same problem myself today - I'm building a component that will scale its text size based on the size of the element. Because renderIntoDocument places your component inside a detached DOM node, it isn't possible to calculate offsetWidth, clientWidth, etc.

Are you testing in a browser or node.js? (EDIT: I see you tagged the question PhantomJS so I'm guessing browser!) If you're in a browser you may be able to render the component into the DOM for real:

React.render(<Slider />, document.body);

If you're worried about test isolation, you can create an IFrame to render the component into, and clean that up afterwards:

beforeEach(function() {
    this.iframe = document.createElement('iframe');

React.render(<Slider />, this.iframe.contentDocument.body);

afterEach(function() {

Then call this.iframe.contentDocument.body.querySelectorAll('.track') to get the HTML Element and run your assertions against it (This is a plain HTML element, not a React component, so use the standard APIs to query it).