Exploring Typescript Reflection: The Importance of Required Parameters and Default Values

Question

Exploring Typescript Reflection: The Importance of Required Parameters and Default Values

In summary: Is there a method to determine whether a typescript parameter is mandatory and/or has a preset value?

Expanding further: Imagine I have the code snippet below:

//Foo.ts
class Bar {
    foo(required:string,defaultValue:number=0,optional?:boolean) {
        ...
    }
}

I am interested in retrieving the following details for each parameter:

The name
The data type
Is it obligatory?
Does it come with a default value?

While I have effectively utilized method decorators alongside the TypeScript reflection API to obtain the data types of the parameters, as well as referenced this approach to retrieve their names, I have not yet discovered a technique to identify if a parameter is mandatory or has a default value.

I acknowledge that the typescript compiler can be accessed from within typescript itself. Therefore, I am curious if there exists a means to utilize the parse tree of the compiler to ascertain whether a parameter is compulsory and/or has a predefined value?

How would this process unfold?

reflection typescript optional-parameters

Answer 1

Answer №1

If you're looking to start from scratch...

At a high level, one approach is:

Determine how to obtain the SourceFile node using your file's compiler API, which entails some explanation.
Utilize the API's forEachChild function to iterate through all nodes in the file and locate the node with a kind of SyntaxKind.ClassDeclaration and a .name property containing the text Bar.
Subsequently, iterate through all the class's children by employing the API's forEachChild function again, identifying those with a kind of SyntaxKind.MethodDeclaration and a .name property holding the text foo.
To access the parameters, loop over the method node's parameters property.
For each parameter node, retrieve the name by calling .getText() on the .name property.

Determine if the parameter is optional by:

 const parameterDeclaration = parameterNode as ts.ParameterDeclaration;
 const isOptional = parameterDeclaration.questionToken != null || parameterDeclaration.initializer != null || parameterDeclaration.dotDotDotToken != null;

Alternatively, utilize the TypeChecker's isOptionalParameter method.

To find its default expression, simply inspect the initializer property:
```
propertyDeclaration.initializer;
```
To determine the type, employ the TypeChecker's getTypeOfSymbolAtLocation method and provide the symbol of the node... this can get complex due to variations with union types, so I won't delve into it further.

Avoid starting from scratch...

I've developed a wrapper for the TypeScript compiler API. Simply use this code snippet with ts-simple-ast (note: Previously mentioned my previous ts-type-info library, but ts-simple-ast is more robust):

import { Project } from "ts-morph";

// explore setup details here:
// https://ts-morph.com/setup/adding-source-files
const project = new Project({ tsConfigFilePath: "tsconfig.json" });
const sourceFile = project.getSourceFileOrThrow("src/Foo.ts");
const method = sourceFile.getClassOrThrow("Bar").getInstanceMethodOrThrow("foo");

Once you have the method, gathering necessary information from its parameters is straightforward:

console.log(method.getName()); // foo

for (const param of method.getParameters()) {
    console.log(param.getName());
    console.log(param.getType().getText());
    console.log(param.isOptional());
    console.log(param.getInitializer() != null);
}

Answer 2

If you're looking to start from scratch...

At a high level, one approach is:

Determine how to obtain the SourceFile node using your file's compiler API, which entails some explanation.
Utilize the API's forEachChild function to iterate through all nodes in the file and locate the node with a kind of SyntaxKind.ClassDeclaration and a .name property containing the text Bar.
Subsequently, iterate through all the class's children by employing the API's forEachChild function again, identifying those with a kind of SyntaxKind.MethodDeclaration and a .name property holding the text foo.
To access the parameters, loop over the method node's parameters property.
For each parameter node, retrieve the name by calling .getText() on the .name property.

Determine if the parameter is optional by:

 const parameterDeclaration = parameterNode as ts.ParameterDeclaration;
 const isOptional = parameterDeclaration.questionToken != null || parameterDeclaration.initializer != null || parameterDeclaration.dotDotDotToken != null;

Alternatively, utilize the TypeChecker's isOptionalParameter method.

To find its default expression, simply inspect the initializer property:
```
propertyDeclaration.initializer;
```
To determine the type, employ the TypeChecker's getTypeOfSymbolAtLocation method and provide the symbol of the node... this can get complex due to variations with union types, so I won't delve into it further.

Avoid starting from scratch...

I've developed a wrapper for the TypeScript compiler API. Simply use this code snippet with ts-simple-ast (note: Previously mentioned my previous ts-type-info library, but ts-simple-ast is more robust):

import { Project } from "ts-morph";

// explore setup details here:
// https://ts-morph.com/setup/adding-source-files
const project = new Project({ tsConfigFilePath: "tsconfig.json" });
const sourceFile = project.getSourceFileOrThrow("src/Foo.ts");
const method = sourceFile.getClassOrThrow("Bar").getInstanceMethodOrThrow("foo");

Once you have the method, gathering necessary information from its parameters is straightforward:

console.log(method.getName()); // foo

for (const param of method.getParameters()) {
    console.log(param.getName());
    console.log(param.getType().getText());
    console.log(param.isOptional());
    console.log(param.getInitializer() != null);
}

Exploring Typescript Reflection: The Importance of Required Parameters and Default Values

Answer №1

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