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<!DOCTYPE html> <html> <body> <p>删除注释（在行的开头）以测试每个案例：</p> <p id="demo"></p> <script> var x = 5; //x = 5 + 7; // x.valueOf() is 12, typeof x is a number //x = 5 + "7"; // x.valueOf() is 57, typeof x is a string //x = "5" + 7; // x.valueOf() is 57, typeof x is a string //x = 5 - 7; // x.valueOf() is -2, typeof x is a number //x = 5 - "7"; // x.valueOf() is -2, typeof x is a number //x = "5" - 7; // x.valueOf() is -2, typeof x is a number //x = 5 - "x"; // x.valueOf() is NaN, typeof x is a number document.getElementById("demo").innerHTML = x.valueOf() + " " + typeof x; </script> </body> </html>