CODEWITHSUNDEEP
javascriptunit-testingchai
Jeanluca Scaljeri
Jeanluca Scaljeri

Reputation: 29129

How to test if something is a function with Chai.should

I want to check if something is a function with Chai.should. So I did

typeof(barfoo).should.equals('function')

DEMO

This results in

AssertionError: expected [Function] to equal 'function'
    at Context.<anonymous> (:73:31)

Can someone explain to me why this doesn't work, because if I simply do

typeof(barfoo)

I get a string function. Although I've solved this now with instanceOf but I really want to understand this

Upvotes: 17

Views: 11191

Answers (3)

Eugene Tsakh
Eugene Tsakh

Reputation: 2879

It's also possible to test with instanceOf:

barfoo.should.be.instanceOf(Function);

Upvotes: 5

Do Async
Do Async

Reputation: 4284

Here is how you can check if barfoo is a function:

barfoo.should.be.a('function');

Upvotes: 36

Bryan Chen
Bryan Chen

Reputation: 46598

You need extra parentheses

(typeof(barfoo)).should.equals('function')

Otherwise your expression is interpreted as

typeof ((barfoo).should.equals('function'))

as you can see, you are calling .should on barfoo instead of typeof barfoo and then get the type of final result and discard it.

This is because typeof it not a function, it is an unary operator[1] and have a lower precedence than Member Access operator (.) and Function Call[2]. So typeof is evaluated last in this expression.

Upvotes: 8

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