Formatting length() is for arrays — on a string it wrecks the expression
In list-formatting expressions length() counts array elements, not characters — feed it a string and the surrounding arithmetic collapses (typically into a negative substring index), so the whole txtContent renders empty; get a string's length as indexOf(str + '^', '^') with a sentinel that cannot occur in the data.
Ve formátovacích výrazech length() počítá prvky pole, ne znaky – na řetězci se okolní aritmetika zhroutí (typicky do záporného indexu substring) a celý txtContent se vykreslí prázdný; délku řetězce získáš jako indexOf(str + '^', '^') se sentinelem, který se v datech nemůže vyskytnout.
Symptom
Stripping the file extension from a name, the "obvious" way:
"txtContent": "=substring([$FileLeafRef], 0, length([$FileLeafRef]) - length([$File_x0020_Type]) - 1)"The element renders empty on every item. No error anywhere — the formatter saves, the rest of the card renders, just this text is blank.
Cause
Per the formatting syntax reference, length "returns the number of elements in an array". Strings are not arrays here; the call does not return the character count, the subtraction produces a nonsense (negative) end index, and substring with a negative bound yields ''.
This differs from every mainstream expression language a developer carries in muscle memory, which is what makes it a trap: the expression looks obviously correct, and nothing flags it.
Fix
Derive length from indexOf over the string with a sentinel appended at the end:
"=indexOf([$FileLeafRef] + '^', '^')"The sentinel's first occurrence is the one you appended, so its index is the string's length. Extension-stripping then works as:
"txtContent": "=substring([$FileLeafRef], 0, indexOf([$FileLeafRef] + '^', '^') - indexOf([$File_x0020_Type] + '^', '^') - 1)"Pick a sentinel that cannot appear in the data ('^' is a reasonable default for file names; '.' is not — see Notes).
Notes
- The same sentinel idiom fixes "is the string empty/whole-string operations" cases where you'd otherwise reach for
length(). - Related trap avoided by the sentinel: truncating at the first
'.'(indexOf(name, '.')) butchers names with dots in the middle ("Analýza – 1. pololetí.xlsx" → "Analýza – 1"). - A number-to-string trim variant of the same idiom:
substring(toString(n) + '.', 0, indexOf(toString(n) + '.', '.'))cuts the decimal part of a computed number (useful becausetoFixed()does not exist in formatting either — it renders as literal text). - When the expression drives
txtContent, failure is a blank text; when it drives an attribute, expect harsher outcomes (non-webhrefdrops the element).