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  • String.prototype.until

String.prototype.until

# Adam Shannon (14 years ago)

Hello all,

I recently ran into a situation where I would like to obtain a substring from the beginning until the first encounter with another substring. This promoted me to write a simple function, called until and I wondered if it would be something to add with the other string extras for ES.next.

It could be defined as acting the same way as the following code:

String.prototype.until = function (needle) { return this.substr(0, this.indexOf(needle)); }

# Michael A. Smith (14 years ago)

Semantically, calling it "until" makes me think that if the "needle" isn't found, it should return the entire haystack. Your example implementation would return an empty string in that case. Also, to keep consistency with other string methods like substr, shouldn't we allow the developer to decide the starting index?

String.prototype.until = function (start, needle) { return "" + (this.substr(start, this.indexOf(needle)) || this); }

(The ["" +] part is probably not necessary, but it makes it easier to see the implementation work in the console.)

Michael A. Smith Web Developer True Action Network (an eBay Company)

# Adam Shannon (14 years ago)

Yes, I see the use for returning the entire string if the needle isn't found. I was also thinking about a dynamic start position, which is why I'd favor something like this.

String.prototype.until = function (needle, start) { start ?? 0; return this.substr(start, this.indexOf(needle)) || this }

It seems weird to call something like str.until("bc", 2), but the other option would be to check if arguments[0] is a number, and if so to set start = arguments[0] and needle arguments[1].

# Axel Rauschmayer (14 years ago)

Isn’t that usually better handled via a regular expression?

One of the use cases for quasis [1][2] is to make it easy to insert literal text into a regular expression. That seems pertinent here. Example:

re`\d+(${localeSpecificDecimalPoint}\d+)?`

The text in the variable localeSpecificDecimalPoint is matched literally by the regular expression produced by re``.

[1] harmony:quasis [2] www.2ality.com/2011/09/quasi-literals.html

# Adam Shannon (14 years ago)

Alex, I'm confused as to what regular expressions would help with in this case. (Over .indexOf) The idea of .util() would be to return a new string which is just a substring, but provided as an "ease of use" to the developer.

The case where I wrote .util() was in parsing out two comma separated values. I only needed the first, as could also be seen with trying to pull the first name (assuming just a first and last name), you could easily call name.until(' ') and get that back.

# Axel Rauschmayer (14 years ago)

/^(.?)needle.$/.exec("foobar_needle")[1] 'foobar_'