trying to understand the concept of being unable to mutate strings/numbers in javascript -

in javascript, can reassign variable like:

var x = 10; x+=10; console.log(x); //prints 20 console 

but if take example of seemingly similar activity unexpected result:

var originalvar = 1;  changemyvar(originalvar);  function changemyvar(myvar) {   myvar += 1000;   return myvar; }  console.log(originalvar); //prints 1 console 

i see 1 in same. passing variable argument function. i'm reassigning value within function. , i'm returning , printing variable. mutation?

primitives function parameters passed value in javascript. therefore myvar in changemyvar function not reference originalvar new variable value of originalvar.