testsAndMisc/python_pkg/js_anki/string.json

[
 {"method":"charAt / s[i]","tier":"core","task":"Get the character at index i","call":"s[i]   // or s.charAt(i)","mutates":false,"returns":"1-char string; s[i] gives undefined out of range, charAt gives ''","example":"return ['abc'[1],'abc'.charAt(5),'abc'[5]];","expected":"[\"b\",\"\",null]","gotcha":"Strings are immutable: s[0] = 'x' silently does nothing."},
 {"method":"charCodeAt","tier":"core","task":"Get the char code of the character at index i","call":"s.charCodeAt(i)","mutates":false,"returns":"number (UTF-16 code unit)","example":"return ['a'.charCodeAt(0),'c'.charCodeAt(0)-'a'.charCodeAt(0)];","expected":"[97,2]","gotcha":"Alphabet index idiom: s.charCodeAt(i) - 97."},
 {"method":"String.fromCharCode","tier":"core","task":"Turn a char code into a character","call":"String.fromCharCode(code)","mutates":false,"returns":"string","example":"return String.fromCharCode(97+2);","expected":"\"c\"","gotcha":"Static method on String, not on a string value."},
 {"method":"split","tier":"core","task":"Split a string into an array by a separator","call":"s.split(sep)","mutates":false,"returns":"array of strings","signature":"s.split(separator, limit)  // split('') = chars","example":"return ['a,b'.split(','),'abc'.split('')];","expected":"[[\"a\",\"b\"],[\"a\",\"b\",\"c\"]]"},
 {"method":"[...s]","tier":"core","task":"Turn a string into an array of characters","call":"[...s]   // or Array.from(s)","mutates":false,"returns":"new array of characters","example":"return [...'abc'];","expected":"[\"a\",\"b\",\"c\"]","gotcha":"Unlike split(''), spread keeps emoji/surrogate pairs whole."},
 {"method":"reverse a string","tier":"core","task":"Reverse a string","call":"[...s].reverse().join('')","mutates":false,"returns":"new string","example":"return [...'abc'].reverse().join('');","expected":"\"cba\"","gotcha":"Strings have no reverse(); go via an array."},
 {"method":"slice","tier":"core","task":"Get s[i..j) as a substring","call":"s.slice(i, j)","mutates":false,"returns":"new string; end exclusive","signature":"s.slice(start, end)  // end exclusive, negatives count from the end","example":"return ['abcdef'.slice(1,3),'abcdef'.slice(-2)];","expected":"[\"bc\",\"ef\"]"},
 {"method":"substring","tier":"core","task":"Substring by two indices (and how it differs from slice)","call":"s.substring(i, j)","mutates":false,"returns":"new string; end exclusive","signature":"s.substring(start, end)  // swaps args if start > end, negatives → 0","example":"return ['abcdef'.substring(3,1),'abcdef'.slice(3,1),'abcdef'.substring(-2)];","expected":"[\"bc\",\"\",\"abcdef\"]","gotcha":"Prefer slice(): substring swaps arguments and treats negatives as 0."},
 {"method":"indexOf","tier":"core","task":"Find the index of a substring (-1 if absent)","call":"s.indexOf(sub)","mutates":false,"returns":"first index, or -1","signature":"s.indexOf(searchString, fromIndex)","example":"return ['hello'.indexOf('l'),'hello'.indexOf('z')];","expected":"[2,-1]"},
 {"method":"includes","tier":"core","task":"Check whether a string contains a substring","call":"s.includes(sub)","mutates":false,"returns":"boolean","example":"return 'hello'.includes('ell');","expected":"true","gotcha":"Case-sensitive; a RegExp argument throws."},
 {"method":"startsWith","tier":"core","task":"Check whether a string starts with a prefix","call":"s.startsWith(prefix)","mutates":false,"returns":"boolean","example":"return 'hello'.startsWith('he');","expected":"true"},
 {"method":"endsWith","tier":"core","task":"Check whether a string ends with a suffix","call":"s.endsWith(suffix)","mutates":false,"returns":"boolean","example":"return 'hello'.endsWith('lo');","expected":"true"},
 {"method":"toLowerCase","tier":"core","task":"Lowercase a string","call":"s.toLowerCase()","mutates":false,"returns":"new string","example":"return 'aBc'.toLowerCase();","expected":"\"abc\""},
 {"method":"toUpperCase","tier":"core","task":"Uppercase a string","call":"s.toUpperCase()","mutates":false,"returns":"new string","example":"return 'aBc'.toUpperCase();","expected":"\"ABC\""},
 {"method":"trim","tier":"core","task":"Strip whitespace from both ends","call":"s.trim()","mutates":false,"returns":"new string","example":"return '  a b '.trim();","expected":"\"a b\""},
 {"method":"repeat","tier":"core","task":"Repeat a string n times","call":"s.repeat(n)","mutates":false,"returns":"new string","example":"return 'ab'.repeat(3);","expected":"\"ababab\"","gotcha":"Negative n throws RangeError."},
 {"method":"padStart","tier":"core","task":"Left-pad a string to a length (e.g. zero-pad)","call":"s.padStart(len, '0')","mutates":false,"returns":"new string","signature":"s.padStart(targetLength, padString)","example":"return '5'.padStart(3,'0');","expected":"\"005\""},
 {"method":"padEnd","tier":"core","task":"Right-pad a string to a length","call":"s.padEnd(len, ' ')","mutates":false,"returns":"new string","signature":"s.padEnd(targetLength, padString)","example":"return 'ab'.padEnd(5,'.');","expected":"\"ab...\""},
 {"method":"replace","tier":"core","task":"Replace occurrences of a pattern (first only vs all)","call":"s.replace(pattern, replacement)","mutates":false,"returns":"new string","signature":"s.replace(patternOrString, replacementOrFn)","example":"return ['aaa'.replace('a','b'),'aaa'.replace(/a/g,'b')];","expected":"[\"baa\",\"bbb\"]","gotcha":"A string pattern replaces only the FIRST match. Use /x/g or replaceAll."},
 {"method":"replaceAll","tier":"core","task":"Replace ALL occurrences of a substring","call":"s.replaceAll(sub, replacement)","mutates":false,"returns":"new string","signature":"s.replaceAll(patternOrString, replacementOrFn)","example":"return 'aaa'.replaceAll('a','b');","expected":"\"bbb\"","gotcha":"A RegExp argument must have the g flag or it throws."},
 {"method":"at","tier":"core","task":"Get the LAST character (or nth from the end)","call":"s.at(-1)","mutates":false,"returns":"1-char string, or undefined","example":"return ['abc'.at(-1),'abc'.at(0)];","expected":"[\"c\",\"a\"]"},
 {"method":"match","tier":"core","task":"Find all regex matches in a string","call":"s.match(/\\d+/g)","mutates":false,"returns":"array of matches, or null if none","example":"return ['a1b22'.match(/\\d+/g),'abc'.match(/\\d+/g)];","expected":"[[\"1\",\"22\"],null]","gotcha":"Without the g flag you get a single match object, not all matches. null → use ?? []."},
 {"method":"matchAll","tier":"core","task":"Iterate all regex matches including capture groups","call":"[...s.matchAll(/(\\d)(\\w)/g)]","mutates":false,"returns":"iterator of match arrays","example":"return [...'1a2b'.matchAll(/(\\d)(\\w)/g)].map((m)=>m[1]+m[2]);","expected":"[\"1a\",\"2b\"]","gotcha":"The regex must have the g flag or it throws."},
 {"method":"trimStart","tier":"rare","task":"Strip whitespace from the START only","call":"s.trimStart()","mutates":false,"returns":"new string","example":"return '  a '.trimStart();","expected":"\"a \""},
 {"method":"trimEnd","tier":"rare","task":"Strip whitespace from the END only","call":"s.trimEnd()","mutates":false,"returns":"new string","example":"return '  a '.trimEnd();","expected":"\"  a\""},
 {"method":"lastIndexOf","tier":"rare","task":"Find the LAST index of a substring","call":"s.lastIndexOf(sub)","mutates":false,"returns":"last index, or -1","example":"return 'hello'.lastIndexOf('l');","expected":"3"},
 {"method":"substr","tier":"rare","task":"Substring by start index and LENGTH (deprecated)","call":"s.substr(start, length)","mutates":false,"returns":"new string","signature":"s.substr(start, length)  // deprecated; use slice","example":"return 'abcdef'.substr(1,3);","expected":"\"bcd\"","gotcha":"Second arg is a length, not an end index. Deprecated."},
 {"method":"concat","tier":"rare","task":"Concatenate strings with a method call","call":"s.concat(a, b)","mutates":false,"returns":"new string","example":"return 'a'.concat('b','c');","expected":"\"abc\"","gotcha":"Just use + or a template literal."},
 {"method":"search","tier":"rare","task":"Find the index of a regex match","call":"s.search(/c/)","mutates":false,"returns":"index, or -1","example":"return 'abc'.search(/c/);","expected":"2"},
 {"method":"localeCompare","tier":"rare","task":"Compare two strings for sorting","call":"a.localeCompare(b)","mutates":false,"returns":"negative / 0 / positive number","example":"return ['b','a','c'].sort((x,y)=>x.localeCompare(y));","expected":"[\"a\",\"b\",\"c\"]","gotcha":"Locale-aware and slower; for plain code-unit order use a < b ? -1 : 1."},
 {"method":"codePointAt","tier":"rare","task":"Get the full Unicode code point at index i","call":"s.codePointAt(i)","mutates":false,"returns":"number","example":"return '😀'.codePointAt(0);","expected":"128512","gotcha":"charCodeAt would return only half of the surrogate pair."},
 {"method":"normalize","tier":"rare","task":"Normalize Unicode (e.g. strip accents with NFD)","call":"s.normalize('NFD')","mutates":false,"returns":"new string","example":"return 'é'.normalize('NFD').length;","expected":"2"}
]