ReactJS – React Unit Test, Unselected “Push” of undefined reaction routers

_viewCompany(companyId) {
 this.context.router.push(`/admin/companies/${companyId}`);
}

it('should call _viewCompany', () => {
 jest. spyOn(CompaniesList.prototype,'_viewCompany');
 wrapper = mount();
 const viewButton = wrapper.find('.btn-info'). at(0);
 viewButton.simulate('click');
 expect(CompaniesList.prototype._viewCompany).toHaveBeenCalled();
 });

function Router () {
 this.router = [];

 this.push = function(a) {
 this.router.push( a);
 };

 this.get = function(index){
 return this.router[index];
 }
 this.length = function(){
 return this.router.length;
 }
}

function History () {
 this.history = []; 

 this.push = function(a) {
 this.history.push(a);
 };

 this.get = function( index){
 return this.history[index];
 }
 this.length = function(){
 return this.history.length;
 }
}

it('should call _viewCompany', () => {
 jest.spyOn(CompaniesList.prototype,'_viewCompany');
 wrapper = mount(< CompaniesList {...props}/>,{context:{router: new 
 Router(), history: new History()}});
 co nst viewButton = wrapper.find('.btn-info').at(0);
 viewButton.simulate('click');
 expect(CompaniesList.prototype._viewCompany).toHaveBeenCalled(); 
 expect(wrapper.context().router.length()).toEqual('1');
});

I have a component that uses React Router, as shown below:

 _viewCompany(companyId) {
 this.context.router.push(`/admin/companies/${companyId}`);
}

It works well in the component, but I encountered an error while writing a test for the component. This is the test that caused the problem (I use enzyme, jest, sinon):

it('should call _viewCompany', () => {
 jest.spyOn(CompaniesList.prototype,'_viewCompany');
 wrapper = mount();
 const viewButton = wrapper.find ('.btn-info').at(0);
 viewButton.simulate('click');
 expect(CompaniesList.prototype._viewCompany).toHaveBeenCalled();
 }) ;

This test returns an error, stating the following:

Undefined property’push’ cannot be read

What can I do to simulate it or for testing Create an empty function?

You need to pass the context object when installing the component.

function Router () { 
 this.router = [];

 this.push = function(a) {
 this.router.push(a);
 };

 this.get = function(index){
 return this.router[index];
 }
 this.length = function(){
 return this. router.length;
 }
}

function History () {
 this.history = [];

 this.push = function(a) {
 this.history.push(a);
 };

 this.get = function(index){
 return this.history[ index];
 }
 this.length = function(){
 return this.history.length;
 }
}

it ('should call _viewCompany', () => {
 jest.spyOn(CompaniesList.prototype,'_viewCompany');
 wrapper = mount(,{context :{router: new 
 Router(), history: new History())));
 const viewButton = wrapper.find('.btn-info').at(0);
 viewButton.simulate('click');
 expect(CompaniesList.prototype._viewCompany).toHaveBeenCalled();
 expect(wrapper.context().router.length()).toEqual(' 1');
});

You can even test the context object. Just like I did above.