var str1 = new String("赤色") var str2 = new String("赤色") function str_red() { return this.fontcolor('red') } function fake_red() { return this.fontcolor('crimson') } String.prototype.red = str_red document.write("str1=",str1.red(),"<BR>") document.write("str2=",str2.red(),"<BR>") str1.red = fake_red document.write("ここでstr1のstr_red()をfake_red()に置き換えた<BR>") document.write("str1=",str1.red(),"<BR>") document.write("str2=",str2.red())