Creating JavaScript prototype

Question

I'm trying to create a simple prototype in JavaScript that removes a character from a string (like Python's strip). This may well exist already in JavaScript, but I can't find it and this is more of an exercise for me than a replacement for something that already exists.

function strip(chars) {
    this.replace(chars,"");
}

String.prototype.strip=strip;

var wrong = "Hellow";

alert(wrong.strip("w"));

When I run this code, the alert says "Undefined". My suspicion is the this. in the function but not sure. I've also made a fiddle here: http://jsfiddle.net/jdb1991/rapxM/

Thanks

Rob W · Accepted Answer · 2012-01-25 17:59:04Z

5

You have to specify return, so that the result of the operation is returned:

function strip(chars) {
   return this.replace(chars,"");
}

Working demo: http://jsfiddle.net/rapxM/1/

Notice that your code does not remove all characters, it does only remove the first occurrence. If you want to remove all characters, use:

function strip(chars) {
   return this.split(chars).join('');
}

.split splits the string in smaller pieces, based on the given argument (returned value: array).
.join() is an array method, which concatenates the array (returned value: string)

answered Jan 25, 2012 at 17:59

Rob W

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3 Comments

joedborg Over a year ago

Thanks! I knew about the one occurrence, just wanted to get prototypes working. +1 for example and will set as answer when I hit the timer.

Jan Pöschko Over a year ago

Just note that Python's strip doesn't remove occurrences from the middle of the string (docs.python.org/library/stdtypes.html#str.strip). Don't know what @jdborg actually wants.

Rob W Over a year ago

If he wants to emulate Python's strip method, he can either switch to a Regular Expression, or trim the characters in two loops.

Jan Pöschko · Accepted Answer · 2012-01-25 18:14:09Z

If you want to have the same behavior as Python's strip, you should only remove characters from the start and end of the string. Here is a solution with regular expressions that accomplishes this:

function strip(chars, wholeString) {
    if (wholeString)
        chars = "(" + chars + ")*";
    else
        chars = "[" + chars + "]*";
    var pattern = "^" + chars + "|" + chars + "$";
    return this.replace(new RegExp(pattern, "g"),"");
}

If wholeString is set to true, it will treat chars as a block of characters that have to occur together (your question seemed to ask for that), while if not set (the default), each character in chars will be removed on its own (like in Python). Use it like this:

String.prototype.strip=strip;

var str = "ababccabcccbbab";
document.write(str.strip("ab"));       // -> "ccabccc"
document.write(str.strip("ab", true)); // -> "ccabcccbb"

Note that ab is not removed from the middle of str. In the first call, a and b are treated separately, so bb is also stripped.

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