function doSomething(val) {

  "use strict"; 

  x = val + 10;

}

​

console.log(doSomething(10));

​

// It will throw an error because `x` was not defined and it is being set to

// some value in the global scope, which isn't allowed with `use strict`.

// The small change below fixes the error being thrown:

​

function doSomething(val) {

  "use strict"; 

  var x = val + 10;

  return x;

}

​

console.log(doSomething(10));