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    1.0.7 • Public • Published


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    What is this?

    Right now the type checker for Typescript API doesn't expose methods for checking assignability and building types. See issue #9879 and #29432 on the Typescript github repository.

    To fill in the gap while this issue is being discussed this library aims to provide the most essential helper functions for working with types in Typescript.

    Furthermore, this library can help you construct types (called SimpleType) which can be serialized and easy analyzed.

    This library has more than 35000 tests comparing results to actual Typescript diagnostics (see test-types.ts).


    npm install ts-simple-type

    How to use

    The API is very simple. For example if you want to check if Typescript type typeB is assignable to typeA, you can use the following function.

    import { isAssignableToType } from "ts-simple-type";
    const isAssignable = isAssignableToType(typeA, typeB, typeChecker);


    To make it easier to work with typescript types this library works by (behind the curtain) converting them to the interface SimpleType. Most functions in this library work with both SimpleType and the known and loved Typescript-provided Type interface. This means that you can easily create a complex type yourself and compare it to a native Typescript type. It also means that you can use this library to serialize types and even compare them in the browser.

    The SimpleType interface can be used to construct your own types for typechecking.

    import { SimpleType, typeToString, isAssignableToType, isAssignableToValue } from "ts-simple-type";
    const colors: SimpleType = {
      kind: "UNION",
      types: [
        { kind: "STRING_LITERAL", value: "RED" },
        { kind: "STRING_LITERAL", value: "GREEN" },
        { kind: "STRING_LITERAL", value: "BLUE" }
    > "RED" | "GREEN" | "BLUE"
    isAssignableToType(colors, { kind: "STRING_LITERAL", value: "YELLOW" })
    > false;
    isAssignableToValue(colors, "BLUE")
    > true;
    isAssignableToValue(colors, "PINK")
    > false;

    More examples

    const typeA = checker.getTypeAtLocation(nodeA);
    const typeB = checker.getTypeAtLocation(nodeB);
      For this example, let's say:
      - typeA is number
      - typeB is string[]
    // typeToString
    > "number"
    > "string[]"
    // isAssignableToType
    isAssignableToType(typeA, typeB, checker)
    > false
    isAssignableToType(typeA, { kind: "NUMBER" }, checker)
    > true
    isAssignableToType(typeB, { kind: "ARRAY", type: {kind: "STRING"}}, checker)
    > true
      { kind: "STRING" },
      { kind: "STRING_LITERAL", value: "hello"})
    > true
    // isAssignableToPrimitiveType
    isAssignableToPrimitiveType(typeA, checker)
    > true
    isAssignableToPrimitiveType(typeB, checker)
    > false
    isAssignableToPrimitiveType({ kind: "ARRAY", type: {kind: "STRING"} })
    > false
    // isAssignableToSimpleTypeKind
    isAssignableToSimpleTypeKind(typeA, "NUMBER", checker)
    > true
    isAssignableToSimpleTypeKind(typeB, "BOOLEAN", checker)
    > false
    isAssignableToSimpleTypeKind(typeB, ["STRING", "UNDEFINED"], checker)
    > true
    // isAssignableToValue
    isAssignableToValue(typeA, 123, checker)
    > true
    isAssignableToValue(typeA, "hello", checker)
    > false
    isAssignableToValue(typeB, true, checker)
    > false
    // toSimpleType
    toSimpleType(typeA, {checker})
    > { kind"NUMBER" }
    toSimpleType(typeB, {checker})
    > { kind"ARRAY", type{ kind"NUMBER" } }

    API Documentation

    For functions that take either a native Typescript Type or a SimpleType the TypeChecker is only required if a Typescript Type has been given to the function.


    isAssignableToType(typeA: Type | SimpleType, typeB: Type | SimpleType, checker?: TypeChecker): boolean

    Returns true if typeB is assignable to typeA.


    isAssignableToPrimitiveType(type: Type | SimpleType, checker?: TypeChecker): boolean

    Returns true if type is assignable to a primitive type like string, number, boolean, bigint, null or undefined.


    isAssignableToSimpleTypeKind(type: Type | SimpleType, kind: SimpleTypeKind | SimpleTypeKind[], checker?: TypeChecker, options?: Options): boolean

    Returns true if type is assignable to a SimpleTypeKind.

    • options.matchAny (boolean): Can be used to allow the "any" type to match everything.


    isAssignableToValue(type: SimpleType | Type, value: any, checker?: TypeChecker): boolean

    Returns true if the type of the value is assignable to type.


    typeToString(type: SimpleType): string

    Returns a string representation of the simple type. The string representation matches the one that Typescript generates.


    toSimpleType(type: Type | Node, checker: TypeChecker): SimpleType

    Returns a SimpleType that represents a native Typescript Type.


    npm i ts-simple-type

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