How to use dependency injection for testing?

I have piece of code in js like this:

var obj = (function(){ var stateObj = {key:"privateValue"}; return { getState: function() { return stateObj.key; }, publicFn : function(){ //do some operation with stateObj if(getState() == "test") { //. . . . } } } }());

and I tested the code like this:

//test case sandbox.stub(obj.getState,"test") assertItShouldGoInsideIfLoop(obj.publicFn())

However on the code review, my team lead said, this is wrong and he asked me to use Dependency injection for these cases.

I really don't know why the above approach is wrong or even why one should use DI.

-------------Problems Reply------------

If I'm interpreting this correctly, the issue here is that your test for the value of calling publicFn depends on the value of stateObj. The entire idea behind dependency injection is that you provide some means to supply these values in your tests in order to decouple the data from the behavior of the function under test.

var obj = (function(){
return {
setStateObj: function(stateObj) {
this.stateObj = stateObj;
getState: function() {
return this.stateObj.key;
publicFn : function(){
//do some operation with stateObj
if(getState() == "test") {
//. . . .

Now we can use setStateObj to set the state as needed in our tests instead of stubbing the value, which can be dangerous:

obj.setStateObj({ key: 'test' })

obj.setStateObj({ key: 'blah' })

So why is this preferred to stubbing getState? Say we comment the contents of getState:

getState: function() {
// return this.stateObj.key;
}, // returns nothing, but you're stubbing it to return "test" anyway!

Clearly, this function will not work, but with your stub, it will! So you'll have passing tests despite the non-working code!

Here's another example that makes another case for dependency injection. Say I have the following function:

function getDayOfWeek() {
var date = new Date();
var dayNames = ['Sunday','Monday','Tuesday','Wednesday','Thursday','Friday','Saturday'];
return dayNames[date.getDay()];

This getDayOfWeek function is dependent on date. This would be a nightmare to test since we have no control over date's value. If we rewrite this function with a way to supply a date value (i.e. inject the dependency), we can check test the function for any fixed date easily:

function getDayOfWeek(date) {
date = date || new Date();
var dayNames = ['Sunday','Monday','Tuesday','Wednesday','Thursday','Friday','Saturday'];
return dayNames[date.getDay()];

assertEqual('Wednesday', getDayOfWeek(new Date(2015, 0, 1)));
assertEqual('Thursday', getDayOfWeek(new Date(2015, 0, 2)));
// and so on...

Category:javascript Views:0 Time:2018-07-10

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