Jest/Jasmine .toContain() fails even though matching values are present - node.js

Question

I'm writing a simple React application with a Button component, which looks like this:
import React, { Component } from 'react';
// shim to find stuff
Array.prototype.contains = function (needle) {
for (var i = 0; i < this.length; i++) {
if (this[i] == needle) return true;
}
return false;
};
class Button extends Component {
propTypes: {
text: React.PropTypes.string.isRequired,
modifiers: React.PropTypes.array
}
render() {
return(
<span className={this.displayModifiers()}>{this.props.text}</span>
);
}
displayModifiers() {
const modifiers = this.props.modifiers || ["default"];
if (modifiers.contains("default") ||
modifiers.contains("danger") ||
modifiers.contains("success")) {
// do nothing
} else {
// add default
modifiers.push("defualt");
}
var classNames = "btn"
for (var i = 0; i < modifiers.length; i++) {
classNames += " btn-" + modifiers[i]
}
return(classNames);
}
}
export default Button;
I then wrote this to test it:
it("contains the correct bootstrap classes", () => {
expect(mount(<Button modifiers={["flat"]}/>).html()).toContain("<span class=\"btn btn-flat btn-default\"></span>");
});
That code should pass, but I receive the following error message:
expect(string).toContain(value)
Expected string:
"<span class=\"btn btn-flat btn-defualt\"></span>"
To contain value:
"<span class=\"btn btn-flat btn-default\"></span>"
at Object.it (src\__tests__\Button.test.js:42:293)
Any ideas why this is not passing?

Answer 1

From the docs:
Use .toContain when you want to check that an item is in a list.
To test strings you should use toBe or toEqual
it("contains the correct bootstrap classes", () => {
expect(mount(<Button modifiers={["flat"]}/>).html()).toBe("<span class=\"btn btn-flat btn-default\"></span>");
});
But there is a better way of testing the output rendered components: snapshots.
it("contains the correct bootstrap classes", () => {
expect(mount(<Button modifiers={["flat"]}/>).html()).toMatchSnapshot();
});
Note that you will need enzymeToJson for snapshot testing using enzyme.