selfprint.js

A JavaScript program that prints itself

How it works

The idea is to implement a JS program that prints itself on runtime; console.log command will be used for output..

1. Initially a program of such kind can have the following view: s = "<text>"; console.log('s = ' + '<"-symbol>' + s + '<"-symbol>' + s) The main trick is that <text> value includes everything after second double quotes symbol: ; console.log(... etc. We can't use double quotes in <text> value. If we try to escape those, it won't lead us to expected result: <text> string will include escaped values, but printed output wont't include characters used in escaping. So we can't use double quote charater at any position after enclosing quote of s="<text>" and inside <text> (which is the same sequence of characters).

2. The answer is to use unicode escaped characters in strings, " - \u0022, ' - \u0027, \ - \u005c. This will help us to avoid issue related to nested quotes of the same type. But this don't solve the other problem - output of slash character.

3. Let f() be the function that outputs any unicode character from given code. f('0022') -> ", f('005c') -> , etc. The f's body uses eval to process character code.

    f = function(a) {
        return eval('"\\u' + a + '"');
    };
   

4. The program will have the following view: f = function(a) {...}; s = <text>; console.log('<f-output> s = ' + f('0022') + s + f('0022') + s').

    <f-output> = 'f = function(a) {return eval(' + f('0027') + f('0022') + f('005c') + f('005c') + 'u' + f('0027') + ' + a + ' + f('0027') + f('0022') + f('0027') + ');};'
   

5. Finally, <text> value should be specified.

    <text> = '; console.log('f = function(a) {return eval(' + f('0027') + f('0022') + f('005c') + f('005c') + 'u' + f('0027') + ' + a + ' + f('0027') + f('0022') + f('0027') + ');}; s = ' + f('0022') + s + f('0022') + s);'
   

6. Formated text of program. New line characters should be removed.

    f = function(a) {
        return eval('"\\u' + a + '"');
    };
    s = "; console.log('f = function(a) {return eval(' + f('0027') + f('0022') + f('005c') + f('005c') + 'u' + f('0027') + ' + a + ' + f('0027') + f('0022') + f('0027') + ');}; s = ' + f('0022') + s + f('0022') + s);";
    console.log('f = function(a) {return eval(' + f('0027') + f('0022') + f('005c') + f('005c') + 'u' + f('0027') + ' + a + ' + f('0027') + f('0022') + f('0027') + ');}; s = ' + f('0022') + s + f('0022') + s);