Non-coercible objects

noncoercible.js

function nonCoercible(val) {
  if (val == null) {
    throw TypeError('nonCoercible shouldn\'t be called with null or undefined');
  }
  const res = Object(val);
  res[Symbol.toPrimitive] = () => {
    throw TypeError('Trying to coerce non-coercible object');
  }
  return res;
};

// USAGE

// objects
const foo = nonCoercible({foo: 'foo'});

foo * 10;                   // throws TypeError: Trying to coerce non-coercible object
foo + 'evil';               // throws TypeError: Trying to coerce non-coercible object
nonCoercible(['foo'])[0];   // "foo"


// Be sure you understand how it works for Number, String, Boolean and Symbol types
// You should cast value using `toString` or `valueOf` when you want to use it with primitive types
// Remember that nonCoercible always returns an object

// strings
const bar = nonCoercible('bar');

bar + '1';                  // throws TypeError: Trying to coerce non-coercible object
bar.toString() + 1;         // bar1
bar === 'bar';              // false
bar.toString() === 'bar';   // true
bar == 'bar';               // throws TypeError: Trying to coerce non-coercible object

// numbers
const baz = nonCoercible(1);

baz == 1;                   // throws TypeError: Trying to coerce non-coercible object
baz === 1;                  // false
baz.valueOf() === 1;        // true

// booleans
const qux = nonCoercible(true);

qux == true;                // throws TypeError: Trying to coerce non-coercible object
qux === true;               // false
qux.valueOf() === true;     // true

// symbols
const xyzzy = Symbol('xyzzy');
const courge = nonCoercible(xyzzy);

courge === xyzzy;           // false
courge.valueOf() === xyzzy; // true
courge == xyzzy;            // true