Typescript: Verifying if a parameter matches a specific type

Here are my constructor implementations:

constructor(position: FlagPosition, flag: string);
constructor(position: FlagPosition, flag: Expression<any> | string) {
    this.position = position;
    //TODO: Check if flag type is a string or an Expression:
    //if flag is a string
    //    this(position, ExpressionUtils.template(Object.class, flag));
    //else
    //    this.flag = flag;
}

I'm unsure how to determine whether the flag parameter is a string or an Expression<any>.

Any suggestions?

typescripttypescript2.0

Answer №1

It is important to differentiate between the transpilation process from TypeScript to JavaScript and the behavior at runtime.

flag: Expression<any> | string

During the transpilation phase, TypeScript will check if you are passing a string or an 'Expression' when calling the function. It ensures this during translation to JavaScript....

Meanwhile,

//TODO: Check flag type:
//if flag is a string
//    this(position, ExpressionUtils.template(Object.class, flag));
//else
//    this.flag = flag;

This code snippet affects runtime behavior. At runtime, 'Expression' (an interface) no longer exists as JavaScript does not recognize interfaces.

Therefore, while testing the parameter type, within the constructor body, consider it simply as an Object with properties that align with your interface.

Answer №2

One possible approach is to

if (typeof flag === "string") {
//
} else if (name instanceof test) {
//
}

`

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