First name: <input data-bind="value: firstName" />
Last name: <input data-bind="value: lastName" />
Full name: <strong data-bind="text: fullName"></strong>

function AppViewModel() {
    this.firstName = ko.observable("Bert");
    this.lastName = ko.observable("Bertington");
    
    this.fullName = ko.computed(function(){
        return this.firstName() + " " + this.lastName();
    },this);
}

// Activates knockout.js
ko.applyBindings(new AppViewModel());

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about this , I have described in detail in JavaScript 's this pointing problem depth parsing , so I won't repeat it here.

specific to this question, it's about the API, of ko.computed () . Take a look at the excerpt from official document :

Managing 'this'

The second parameter to ko.computed (the bit where we passed this in the above example) defines the value of this when evaluating the computed observable. Without passing it in, it would not have been possible to refer to this.firstName () or this.lastName () . Experienced JavaScript coders will regard this as obvious, but if you're still getting to know JavaScript it might seem strange. (Languages like C-sharp and Java never expect the programmer to set a value for this , but JavaScript does, because its functions themselves aren't part of any object by default.)

The second parameter of ko.computed specifies the this when executing the computed viewer function.

so the this passed in ko.computed (function () {.}, this) is the this used in function () {.} , that is, the this of this.firstName () and this.secondName () . That is, this in the AppViewModel scope.

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this is actually the basis of js functions. You need a red treasure book (Javascript Advanced programming), Function section

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1.) To understand where the this points here, you only need to understand what happens after the function is used as the constructor, that is, after the new keyword
2. In fact, the this here points to the object from AppViewModel new. You can refer to https://developer.mozilla.org.
3.. An anonymous function is a function, but this function does not need to be called anywhere else, so it does not need to have a name. In JavaScript, the function is a first-class citizen and an object, and can be passed

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as an argument.

  
 Brother that function is called self-executing function format (function () {}) (), which is a general format. Now there is es6, so it can also be written as: 
 (() = > {}) () arrow function. As for what you said about this pointing, I think you have to systematically learn that this points to window, to the current object, and to the current function. The this in the callback function points to window.