How to spy on dynamically called function - mocha.js

Question

I'm working with Javascript, Mocha, Chai and SinonJs. What I'm trying to do is to spy on a method "equals" that is called dynamically by another method "compareUsingOperator". The following code shows my class "StringComparator" with the two methods "equals" and "compareUsingOperator".
export default class StringComparator {
private compareFunctionList: {[key in "=" | "!=" | "<>" | "<" | "<=" | ">" | ">="]: Function};
public constructor() {
this.compareFunctionList = {
"=" : this.equals,
"!=" : this.notEquals,
"<>" : this.notEquals,
"<" : this.lessThan,
"<=" : this.lessThanOrEquals,
">" : this.greaterThan,
">=" : this.greaterThanOrEquals,
};
}
public equals(value1: string, value2: string): boolean {
if (typeof value1 != "string" || typeof value2 != "string") {
console.log(value1);
console.log(value2);
throw new TypeError("Invalid data type!");
}
return (value1 === value2);
}
public compareUsingOperator(operator: "=" | "!=" | "<>" | "<" | "<=" | ">" | ">=", value1: string, value2: string): boolean {
if (typeof operator != "string" || typeof value1 != "string" || typeof value2 != "string") {
throw new TypeError("Invalid data type!");
}
if (!Object.keys(this.compareFunctionList).includes(operator)) {
throw new ReferenceError("Undefined operator!");
}
const fn: Function = this.compareFunctionList[operator];
return fn.call(this, value1, value2);
}
}
And this is my Unit test:
describe("compareUsingOperator", () => {
context("calls", () => {
comparator.validOperators.forEach((operator) => {
it("the proper function according to the given operator", () => {
sinon.spy(actual, "equals");
actual.compareUsingOperator("=", comparator.value1, comparator.value2);
assert.called(actual.equals);
actual.equals.restore();
});
});
});
});
In my Unit test I set a spy on the "equals" method like that:
sinon.spy(actual, "equals");
"actual" is an instance of "StringComparator".
In the next step I'm calling:
actual.compareUsingOperator("=", comparator.value1, comparator.value2);
In this method the following code is executed:
const fn: Function = this.compareFunctionList[operator];
return fn.call(this, value1, value2);
This is the call to the "equals" method. And this is also my problem. I want to know if "equals" is called or not. I'm using the following assertion:
assert.called(actual.equals);
The answert I get from chai/sinon is:
AssertError: expected equals to have been called at least once but was never called
The expected behavior should be a green test.
What is wrong with my Unit test? With the help of console.log I can prove that "equals" has been called.
Regards
Florian

Answer 1

I've found the answer myself. I had to replace:
const fn: Function = this.compareFunctionList[operator];
return fn.call(this, value1, value2);
with:
const fn: string = this.compareFunctionList[operator];
return this[fn](value1, value2);
And this:
private compareFunctionList: {[key in "=" | "!=" | "<>" | "<" | "<=" | ">" | ">="]: Function};
public constructor() {
this.compareFunctionList = {
"=" : this.equals,
"!=" : this.notEquals,
"<>" : this.notEquals,
"<" : this.lessThan,
"<=" : this.lessThanOrEquals,
">" : this.greaterThan,
">=" : this.greaterThanOrEquals,
};
}
like that:
private compareFunctionList: {[key in "=" | "!=" | "<>" | "<" | "<=" | ">" | ">="]: string};
public constructor() {
this.compareFunctionList = {
"=" : "equals",
"!=" : "notEquals",
"<>" : "notEquals",
"<" : "lessThan"
"<=" : "lessThanOrEquals",
">" : "greaterThan",
">=" : "greaterThanOrEquals",
};
}
Now the class is testable.
Tanks and regards
Florian