ES5 standards mode catches errors that compatibility mode allows.

Consider the following behavior changes when porting code from existing scripts to new scoped scripts using ES5 standards mode:

• ECMAScript5 evaluates the term new Boolean(false) to true. In compatibility mode, it evaluated to false.
• ECMAScript5 throws an EcmaError when a non-existent property is referenced. In compatibility mode no error was thrown.
• ECMAScript5 throws an EcmaError when a non-existent function is called. In compatibility mode, no error was thrown.
• ECMAScript5 correctly handles new lines. In the past, a newline character after a comment was recognized, which is wrong. In this example, in compatibility mode, all three functions are called. In ECMAScript5, only the first function is called.

  var expr = doFoo();  // do foo
             doBar();  // do bar 
             finish();   // all done
   eval(expr);
  

• ECMAScript5 correctly handles postfix increment and decrement. In this example, in compatibility mode, the variable x gets the incremented value, which is wrong.

  var x = gr.limit++;