如何在Javascript中将变量强制转换为另一个变量的类型

Question

I want to be able to cast a variable to the specific type of another. As an example:

function convertToType(typevar, var) {
    return (type typevar)var; // I know this doesn't work
}

so that convertToType(1, "15") returns 15, convertToType("1", 15) returns "15", convertToType(false, "True") returns true, etc.

To reiterate, I want to be able to dynamically cast variables to the types of other variables.

Is this possible?

Answer 1

UPDATE: here's an even more complete example, with tests (requires Node 6+ or some transpilation to ES5): https://github.com/JaKXz/type-convert

You can use the typeof operator to get the right "casting" function:

function convertToTypeOf(typedVar, input) {
  return {
    'string': String.bind(null, input),
    'number': Number.bind(null, input)
    //etc
  }[typeof typedVar]();
}

Try it out here: http://jsbin.com/kasomufucu/edit?js,console

I would also suggest looking into TypeScript for your project.

Answer 2

function convertToType (t, e) {
    return (t.constructor) (e);
}

note the first call when we wanted to convert 15 to a Number, we append a dot (.) to the first parameter

Answer 3

It follows my version of JaKXz answer that is supposedly more efficient by switching on inferred type. In the case of number and boolean the conversion is loose: in case of invalid input it will output NaN and false , respectively. You can work on this by adding your stricter validation and more types.

function convertToTypeOf(typevar, input)
{
  let type = typeof typevar;
  switch(type)
  {
    case "string":
      return String.bind(null, input)();
    case "number":
      return Number.bind(null, input)();
    case "boolean":
      return input == "true" ? true : false;
    default:
      throw "Unsupported type";
  }
}

console.log(convertToTypeOf("string", 1));      // Returns "1"
console.log(convertToTypeOf("string", true));   // Returns "true"
console.log(convertToTypeOf("string", false));  // Returns "false"
console.log(convertToTypeOf(1.2, "1"));         // Returns 1
console.log(convertToTypeOf(1.2, "1.5"));       // Returns 1.5
console.log(convertToTypeOf(true, "false"));    // Returns false
console.log(convertToTypeOf(false, "true"));    // Returns true
console.log(convertToTypeOf(1, "asd"));         // Returns NaN
console.log(convertToTypeOf(true, "asd"));      // Returns false