How to truncate a string in a Golang template

Question

In golang, is there a way to truncate text in an html template?

For example, I have the following in my template:

{{ range .SomeContent }}
 ....
             


        

Answer 1

There are a lot of good answers, but sometimes it's more user-friendly to truncate without cutting words. Hugo offers template function for it. But it's difficult to use outside of Hugo, so I've implemented it:

func TruncateByWords(s string, maxWords int) string {
    processedWords := 0
    wordStarted := false
    for i := 0; i < len(s); {
        r, width := utf8.DecodeRuneInString(s[i:])
        if !isSeparator(r) {
            i += width
            wordStarted = true
            continue
        }

        if !wordStarted {
            i += width
            continue
        }

        wordStarted = false
        processedWords++
        if processedWords == maxWords {
            const ending = "..."
            if (i + len(ending)) >= len(s) {
                // Source string ending is shorter than "..."
                return s
            }

            return s[:i] + ending
        }

        i += width
    }

    // Source string contains less words count than maxWords.
    return s
}

And here is a test for this function:

func TestTruncateByWords(t *testing.T) {
    cases := []struct {
        in, out string
        n       int
    }{
        {"a bcde", "a...", 1},
        {"a b", "a b", 2},
        {"a b", "a b", 3},

        {"a b c", "a b c", 2},
        {"a b cd", "a b cd", 2},
        {"a b cde", "a b...", 2},

        {"  a   b    ", "  a   b...", 2},

        {"AB09C_D EFGH", "AB09C_D...", 1},
        {"Привет Гоферам", "Привет...", 1},
        {"Here are unicode spaces", "Here are...", 2},
    }

    for i, c := range cases {
        got := TruncateByWords(c.in, c.n)
        if got != c.out {
            t.Fatalf("#%d: %q != %q", i, got, c.out)
        }
    }
}