Mocha / Chai expect.to.throw not catching thrown errors

You have to pass a function to expect. Like this:

expect(model.get.bind(model, 'z')).to.throw('Property does not exist in model schema.');
expect(model.get.bind(model, 'z')).to.throw(new Error('Property does not exist in model schema.'));

The way you are doing it, you are passing to expect the result of calling model.get('z'). But to test whether something is thrown, you have to pass a function to expect, which expect will call itself. The bind method used above creates a new function which when called will call model.get with this set to the value of model and the first argument set to 'z'.

A good explanation of bind can be found here.

This question has many, many duplicates, including questions not mentioning the Chai assertion library. Here are the basics collected together:

The assertion must call the function, instead of it evaluating immediately.

assert.throws(x.y.z);      
   // FAIL.  x.y.z throws an exception, which immediately exits the
   // enclosing block, so assert.throw() not called.
assert.throws(()=>x.y.z);  
   // assert.throw() is called with a function, which only throws
   // when assert.throw executes the function.
assert.throws(function () { x.y.z });   
   // if you cannot use ES6 at work
function badReference() { x.y.z }; assert.throws(badReference);  
   // for the verbose
assert.throws(()=>model.get(z));  
   // the specific example given.
homegrownAssertThrows(model.get, z);
   //  a style common in Python, but not in JavaScript

You can check for specific errors using any assertion library:

Node

  assert.throws(() => x.y.z);
  assert.throws(() => x.y.z, ReferenceError);
  assert.throws(() => x.y.z, ReferenceError, /is not defined/);
  assert.throws(() => x.y.z, /is not defined/);
  assert.doesNotThrow(() => 42);
  assert.throws(() => x.y.z, Error);
  assert.throws(() => model.get.z, /Property does not exist in model schema./)

Should

  should.throws(() => x.y.z);
  should.throws(() => x.y.z, ReferenceError);
  should.throws(() => x.y.z, ReferenceError, /is not defined/);
  should.throws(() => x.y.z, /is not defined/);
  should.doesNotThrow(() => 42);
  should.throws(() => x.y.z, Error);
  should.throws(() => model.get.z, /Property does not exist in model schema./)

Chai Expect

  expect(() => x.y.z).to.throw();
  expect(() => x.y.z).to.throw(ReferenceError);
  expect(() => x.y.z).to.throw(ReferenceError, /is not defined/);
  expect(() => x.y.z).to.throw(/is not defined/);
  expect(() => 42).not.to.throw();
  expect(() => x.y.z).to.throw(Error);
  expect(() => model.get.z).to.throw(/Property does not exist in model schema./);

You must handle exceptions that 'escape' the test

it('should handle escaped errors', function () {
  try {
    expect(() => x.y.z).not.to.throw(RangeError);
  } catch (err) {
    expect(err).to.be.a(ReferenceError);
  }
});

This can look confusing at first. Like riding a bike, it just 'clicks' forever once it clicks.

And if you are already using ES6/ES2015 then you can also use an arrow function. It is basically the same as using a normal anonymous function but shorter.

expect(() => model.get('z')).to.throw('Property does not exist in model schema.');

As this answer says, you can also just wrap your code in an anonymous function like this:

expect(function(){
    model.get('z');
}).to.throw('Property does not exist in model schema.');