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I'm new to JavaScript so this is possibly a trivial question:

I'm trying to construct an object that stores a mapping from a set of integers to some of its methods, i.e. something like this:

'use strict';

function Foo() 
 this.funcs = 
 1: this.func1,
 2: this.func2,
 


Foo.prototype.func1 = function() 
 this.prop = 1;


Foo.prototype.func2 = function() 
 this.prop = 2;

I'd then like to be able to call methods of Foo like this:

foo = new Foo();
var func = foo.funcs[1];
func();

But this results in: Cannot set property 'prop' of undefined, i.e. this does not refer to foo.

What's the problem here and is there a better way to implement this?

There are a few ways to achieve this however the better approach would be to bind() each function to the instance of Foo() that is being instantiated by passing this to bind() of each function:

this.func1.bind(this)

That will ensure that this, for func1 and func2 is defined as the instance of Foo(), which will ensure that this.prop can be accessed and assigned as expected:

'use strict';

function Foo() 
 this.funcs = 
 /* Bind the functions to this Foo() instance */
 1: this.func1.bind(this),
 2: this.func2.bind(this),
 


Foo.prototype.func1 = function() 
 this.prop = 1;
 console.log('called func1. this.prop =', this.prop);


Foo.prototype.func2 = function() 
 this.prop = 2;
 console.log('called func2. this.prop =', this.prop);



const foo = new Foo();
var func = foo.funcs[1];
func();

foo.funcs[2]();

Your problem is this line:

var func = foo.funcs[1];

JavaScript determines the value of this based on how a function is called. If you use dot notation, such as foo.funcs[1](); then the value of this will associated with the foo object. But when you run func(), that's just a plain function and this will have the default value of undefined.

It would be worth your time to read the two chapters of You Don't Know JS that discuss this. It should take less than an hour to learn, and you'll be way ahead of most JS programmers once you learn it.

The rules might not make sense until you read the chapter, but they are summarized below:

Determining the this binding for an executing function requires
finding the direct call-site of that function. Once examined, four
rules can be applied to the call-site, in this order of precedence:

Called with new? Use the newly constructed object.

Called with call or apply (or bind)? Use the specified object.

Called with a context object owning the call? Use that context object.

Default: undefined in strict mode, global object otherwise.

Based on the above rules, the code below is the simplest way you could get it to work the way you are expecting it to:

'use strict';

function Foo() 
 this.funcs = 
 1: this.func1,
 2: this.func2,
 


Foo.prototype.func1 = function() 
 this.prop = 1;
 console.log('called func1. this.prop =', this.prop);


Foo.prototype.func2 = function() 
 this.prop = 2;
 console.log('called func2. this.prop =', this.prop);



const foo = new Foo();
foo.funcs[1]();