Question

I made a code that get a type of inputs.All of the inputs that i add to the function have there correct answers (ex:prompt(10) result is : Number). when i tried to add object like this({}) i get Undefined the code that i made is :

function type() {
  var Input = eval (prompt("please enter a value"));
  var Primitive = [Boolean(Input), String(Input), null, undefined, 
  Number(Input)];
 for (i = 0; i < Primitive.length; i++) {
    if (Primitive[i]) {
      console.log (typeof Input)
    }
  }
}
type();

I tried to use instanceof but i get the same result for object which is undefined.

The Expected result when i prompt {} i get Object. note: for Array [] i get object no problem in it .

Answer 1

if you use eval({}) Javascript will treat {} as an empty block, not an object. I (and many developers) highly disencourage the use of eval()

you can escape this using eval("(" + prompt("please enter a value") + ")");

Here's the difference:

var Input = eval (prompt("please enter a value")); 
// if you type {} this will turn into eval({}) wich is the same as eval() which will return "undefined"

Input = eval("(" + prompt("please enter a value") + ")");
//Typing {} will turn into eval(({})) wich makes {} an expression instead of an empty block. thus returning "object"

Answer 2

{} gets interpreted as an (empty) block of code, not an object literal. It has no return value, so it defaults to undefined.

You'd get the same results if you entered ;.

To get an actual object, you need to make sure { is interpreted as part of an expression, not the beginning of a statement, for example by using ({}).

why do i get undefined when i input {} in side a function that get the type of input

2 个答案: