Comment by nikolay

11 years ago

We have. I spent some time optimizing [0] a String.repeat function over at Stackoverflow and I was surprised that many developers today don't know what they are doing, including core team members [1]. Specifically,

    function repeatString(str, len) {
      return Array.apply(null, {
        length: len + 1
      }).join(str).slice(0, len)
    }

[0]: http://stackoverflow.com/questions/202605/repeat-string-java...

[1]: http://stackoverflow.com/questions/202605/repeat-string-java...

7 comments

nikolay

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Macha  11 years ago

So I don't think this is an example of core team members who don't know what you're doing.

This function repeats a string up to a certain length. e.g.

repeatString(foo, 10) = foofoofoof

This could entirely be what the developers needed in their code base.

The fact that it doesn't repeat a string N times, is at best, an example of bad naming.

The issue here is that someone unrelated to the core team grepped the node codebase for the term they wanted and threw a function up as a SO answer without understanding what it did.

  • nikolay  11 years ago

    Allocate and join an array of blanks just to repeat a string is acceptable?!

    • jestar_jokin  11 years ago

      Joining an array of strings is sometimes better than concatenating a string in place, yes.

      Consider the following:

        1: var str = 'foo';
  2: str += 'bar';
  3: str += 'baz';

      At program load, you have three string constants, 'foo', 'bar', and 'baz'. At 1, 'str' becomes a pointer to the string constant 'foo'. At 2, 'str' becomes a pointer to a string 'foobar'. Memory is allocated to store this string. At 3, 'str' becomes a pointer to a stinrg 'foobarbaz'. Memory is allocated to store this string.

      Now consider the following:

        1: var str = ['foo', 'bar', 'baz'].join('');

      At program load, you have three string constants, 'foo', 'bar', and 'baz'. At 1, 'str' becomes a pointer to the string constant 'foobarbaz'. In the array concatenation, the JS interpreter can create a single string buffer large enough to contain the contents of each array element and the separator string * N - 1 (where N is the number of elements), and return a pointer.

      Once you have a large number of elements, or are joining large strings, the performance difference (especially regarding garbage collection) could matter.

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