What is the significance of default parameters in a JavaScript IIFE within a TypeScript module?

Question

What is the significance of default parameters in a JavaScript IIFE within a TypeScript module?

If I were to create a basic TypeScript module called test, it would appear as follows:

module test
{
    export class MyTest {
        name = "hello";
    }
}

The resulting JavaScript generates an IIFE structured like this:

var test;
(function (test) {
    var MyTest = (function () {
        function MyTest() {
            this.name = "hello";
        }
        return MyTest;
    })();
    test.MyTest = MyTest;
})(test || (test = {}));

What I find puzzling is the purpose of the last line within the IIFE that provides arguments for the function:

(test || (test = {}));

Additionally, the resulting function accepts the parameter test:

(function (test) {

I comprehend using parameters to pass in something like a 'jQuery object such as })(jquery);, resulting in the function being able to use an alias like (function ($) {. However, I am struggling to grasp the intent behind (test || (test = {})); and how these arguments operate.

While I see that test.MyTest = MyTest; exposes the public method MyTest, I am uncertain about the reasoning behind (test || (test = {})); and its functionality. Can someone clarify this for me?

javascript typescript iife

Answer 1

Answer №1

This feature allows for the creation of modules with open-ended possibilities.

(function (example) {
    var MyExample = (function () {
        function MyExample() {
            this.name = "hello";
        }
        return MyExample;
    })();
    example.MyExample = MyExample;
})(example || (example = {}));


(function (example) {
    var SecondExample = (function () {
        function SecondExample() {
            this.name = "hello";
        }
        return SecondExample;
    })();
    example.SecondExample= SecondExample;
})(example || (example = {}));

Initially, example is undefined and is then assigned as an empty object {}. Subsequently, it is already defined and can be extended with SecondExample.

Answer 2

This feature allows for the creation of modules with open-ended possibilities.

(function (example) {
    var MyExample = (function () {
        function MyExample() {
            this.name = "hello";
        }
        return MyExample;
    })();
    example.MyExample = MyExample;
})(example || (example = {}));


(function (example) {
    var SecondExample = (function () {
        function SecondExample() {
            this.name = "hello";
        }
        return SecondExample;
    })();
    example.SecondExample= SecondExample;
})(example || (example = {}));

Initially, example is undefined and is then assigned as an empty object {}. Subsequently, it is already defined and can be extended with SecondExample.

Answer 3

Answer №2

Extending an existing object or creating it if necessary is a common method in JavaScript. Let's examine how this process works.

var example;

This line initializes the variable example, ensuring that no ReferenceError will be thrown when referencing it later on. Its value will either be undefined or the current value of example.

(example || (example = {}))

In this snippet, the value of example is passed if it exists (truthy), otherwise a new object is assigned to example and then passed to the function. This approach allows multiple files to extend a shared namespace object instead of replacing it entirely.

Answer 4

Extending an existing object or creating it if necessary is a common method in JavaScript. Let's examine how this process works.

var example;

This line initializes the variable example, ensuring that no ReferenceError will be thrown when referencing it later on. Its value will either be undefined or the current value of example.

(example || (example = {}))

In this snippet, the value of example is passed if it exists (truthy), otherwise a new object is assigned to example and then passed to the function. This approach allows multiple files to extend a shared namespace object instead of replacing it entirely.

Answer 5

Answer №3

Is this explanation helpful?

let example;

function executeIIFE(example) {
    let MyExample = (function () {
        function MyExample() {
            this.title = "hi there";
        }
        return MyExample;
    })();
    example.MyExample = MyExample;
}

example = example || {}
executeIIFE(example);

Because the `let example` declaration is hoisted to the top without reassigning or overwriting an existing `example`, if an `example` variable is already defined in the same scope as the IIFE, it will be used instead of the empty object.

Answer 6

Is this explanation helpful?

let example;

function executeIIFE(example) {
    let MyExample = (function () {
        function MyExample() {
            this.title = "hi there";
        }
        return MyExample;
    })();
    example.MyExample = MyExample;
}

example = example || {}
executeIIFE(example);

Because the `let example` declaration is hoisted to the top without reassigning or overwriting an existing `example`, if an `example` variable is already defined in the same scope as the IIFE, it will be used instead of the empty object.

Answer 7

Answer №4

Expanding on the previously mentioned points and addressing your additional inline queries:

When they mention "open-ended", they are highlighting the capability to continuously add members to the object at a later time. If object creation and extension were limited to a single source file, we could ensure the initialization order and simplify the code. However, having the below code snippets in script1.ts and script2.ts, included in any sequence on a page, makes it impossible to determine if M1 has been initialized when the script is executed.

script1.ts

module M1 {
    export module M2 {
        export function F1(){return 42;}
    }
}

script2.ts

module M1 {
    export module M2 {
        export function F2(){return "test";}
    }
}

Although redeclaring a variable may not be visually appealing, it is harmless. The value defaults to 'undefined' until initialized, triggering the second part of the expression "M1 || (M1 = {})" to execute and return the newly initialized object as an argument for the first encounter, or pass the existing value for subsequent declarations.

I intentionally utilized nested modules in my example to demonstrate that this concept applies not only to variables but also to object properties. The generated code for the "script1" file above (and likewise for "script2") looks like this:

script1.js

var M1;
(function (M1) {
    var M2;
    (function (M2) {
        function F1() {
            return 42;
        }
        M2.F1 = F1;
    })(M2 = M1.M2 || (M1.M2 = {}));
})(M1 || (M1 = {}));

In this case, the nested module M2 acts as the argument for the immediate function as "M1.M2 || (M1.M2 = {}", providing the existing value of M1.M2 if available, or initializing it to an empty object before proceeding. This object property representing module M1.M2 is referenced by the parameter M2 inside the nested function, allowing additional members (exports) to be added onto it.

I trust this explanation clarifies things for you.

Answer 8

Expanding on the previously mentioned points and addressing your additional inline queries:

When they mention "open-ended", they are highlighting the capability to continuously add members to the object at a later time. If object creation and extension were limited to a single source file, we could ensure the initialization order and simplify the code. However, having the below code snippets in script1.ts and script2.ts, included in any sequence on a page, makes it impossible to determine if M1 has been initialized when the script is executed.

script1.ts

module M1 {
    export module M2 {
        export function F1(){return 42;}
    }
}

script2.ts

module M1 {
    export module M2 {
        export function F2(){return "test";}
    }
}

Although redeclaring a variable may not be visually appealing, it is harmless. The value defaults to 'undefined' until initialized, triggering the second part of the expression "M1 || (M1 = {})" to execute and return the newly initialized object as an argument for the first encounter, or pass the existing value for subsequent declarations.

I intentionally utilized nested modules in my example to demonstrate that this concept applies not only to variables but also to object properties. The generated code for the "script1" file above (and likewise for "script2") looks like this:

script1.js

var M1;
(function (M1) {
    var M2;
    (function (M2) {
        function F1() {
            return 42;
        }
        M2.F1 = F1;
    })(M2 = M1.M2 || (M1.M2 = {}));
})(M1 || (M1 = {}));

In this case, the nested module M2 acts as the argument for the immediate function as "M1.M2 || (M1.M2 = {}", providing the existing value of M1.M2 if available, or initializing it to an empty object before proceeding. This object property representing module M1.M2 is referenced by the parameter M2 inside the nested function, allowing additional members (exports) to be added onto it.

I trust this explanation clarifies things for you.

What is the significance of default parameters in a JavaScript IIFE within a TypeScript module?

Answer №1

Answer №2

Answer №3

Answer №4

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