mirror of
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synced 2026-11-10 22:47:02 +01:00
Krzysztof kuhy Rudnicki
e288b073b9
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Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
348 lines
11 KiB
JSON
348 lines
11 KiB
JSON
[
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{
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"method": "charAt / s[i]",
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"tier": "core",
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"task": "Get the character at index i",
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"call": "s[i] // or s.charAt(i)",
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"mutates": false,
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"returns": "1-char string; s[i] gives undefined out of range, charAt gives ''",
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"example": "return ['abc'[1],'abc'.charAt(5),'abc'[5]];",
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"expected": "[\"b\",\"\",null]",
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"gotcha": "Strings are immutable: s[0] = 'x' silently does nothing."
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},
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{
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"method": "charCodeAt",
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"tier": "core",
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"task": "Get the char code of the character at index i",
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"call": "s.charCodeAt(i)",
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"mutates": false,
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"returns": "number (UTF-16 code unit)",
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"example": "return ['a'.charCodeAt(0),'c'.charCodeAt(0)-'a'.charCodeAt(0)];",
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"expected": "[97,2]",
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"gotcha": "Alphabet index idiom: s.charCodeAt(i) - 97."
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},
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{
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"method": "String.fromCharCode",
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"tier": "core",
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"task": "Turn a char code into a character",
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"call": "String.fromCharCode(code)",
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"mutates": false,
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"returns": "string",
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"example": "return String.fromCharCode(97+2);",
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"expected": "\"c\"",
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"gotcha": "Static method on String, not on a string value."
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},
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{
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"method": "split",
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"tier": "core",
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"task": "Split a string into an array by a separator",
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"call": "s.split(sep)",
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"mutates": false,
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"returns": "array of strings",
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"signature": "s.split(separator, limit) // split('') = chars",
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"example": "return ['a,b'.split(','),'abc'.split('')];",
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"expected": "[[\"a\",\"b\"],[\"a\",\"b\",\"c\"]]"
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},
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{
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"method": "[...s]",
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"tier": "core",
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"task": "Turn a string into an array of characters",
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"call": "[...s] // or Array.from(s)",
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"mutates": false,
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"returns": "new array of characters",
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"example": "return [...'abc'];",
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"expected": "[\"a\",\"b\",\"c\"]",
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"gotcha": "Unlike split(''), spread keeps emoji/surrogate pairs whole."
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},
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{
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"method": "reverse a string",
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"tier": "core",
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"task": "Reverse a string",
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"call": "[...s].reverse().join('')",
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"mutates": false,
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"returns": "new string",
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"example": "return [...'abc'].reverse().join('');",
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"expected": "\"cba\"",
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"gotcha": "Strings have no reverse(); go via an array."
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},
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{
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"method": "slice",
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"tier": "core",
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"task": "Get s[i..j) as a substring",
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"call": "s.slice(i, j)",
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"mutates": false,
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"returns": "new string; end exclusive",
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"signature": "s.slice(start, end) // end exclusive, negatives count from the end",
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"example": "return ['abcdef'.slice(1,3),'abcdef'.slice(-2)];",
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"expected": "[\"bc\",\"ef\"]"
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},
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{
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"method": "substring",
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"tier": "core",
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"task": "Substring by two indices (and how it differs from slice)",
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"call": "s.substring(i, j)",
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"mutates": false,
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"returns": "new string; end exclusive",
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"signature": "s.substring(start, end) // swaps args if start > end, negatives → 0",
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"example": "return ['abcdef'.substring(3,1),'abcdef'.slice(3,1),'abcdef'.substring(-2)];",
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"expected": "[\"bc\",\"\",\"abcdef\"]",
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"gotcha": "Prefer slice(): substring swaps arguments and treats negatives as 0."
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},
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{
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"method": "indexOf",
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"tier": "core",
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"task": "Find the index of a substring (-1 if absent)",
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"call": "s.indexOf(sub)",
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"mutates": false,
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"returns": "first index, or -1",
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"signature": "s.indexOf(searchString, fromIndex)",
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"example": "return ['hello'.indexOf('l'),'hello'.indexOf('z')];",
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"expected": "[2,-1]"
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},
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{
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"method": "includes",
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"tier": "core",
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"task": "Check whether a string contains a substring",
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"call": "s.includes(sub)",
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"mutates": false,
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"returns": "boolean",
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"example": "return 'hello'.includes('ell');",
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"expected": "true",
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"gotcha": "Case-sensitive; a RegExp argument throws."
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},
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{
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"method": "startsWith",
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"tier": "core",
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"task": "Check whether a string starts with a prefix",
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"call": "s.startsWith(prefix)",
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"mutates": false,
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"returns": "boolean",
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"example": "return 'hello'.startsWith('he');",
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"expected": "true"
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},
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{
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"method": "endsWith",
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"tier": "core",
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"task": "Check whether a string ends with a suffix",
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"call": "s.endsWith(suffix)",
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"mutates": false,
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"returns": "boolean",
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"example": "return 'hello'.endsWith('lo');",
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"expected": "true"
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},
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{
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"method": "toLowerCase",
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"tier": "core",
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"task": "Lowercase a string",
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"call": "s.toLowerCase()",
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"mutates": false,
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"returns": "new string",
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"example": "return 'aBc'.toLowerCase();",
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"expected": "\"abc\""
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},
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{
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"method": "toUpperCase",
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"tier": "core",
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"task": "Uppercase a string",
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"call": "s.toUpperCase()",
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"mutates": false,
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"returns": "new string",
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"example": "return 'aBc'.toUpperCase();",
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"expected": "\"ABC\""
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},
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{
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"method": "trim",
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"tier": "core",
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"task": "Strip whitespace from both ends",
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"call": "s.trim()",
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"mutates": false,
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"returns": "new string",
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"example": "return ' a b '.trim();",
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"expected": "\"a b\""
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},
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{
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"method": "repeat",
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"tier": "core",
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"task": "Repeat a string n times",
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"call": "s.repeat(n)",
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"mutates": false,
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"returns": "new string",
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"example": "return 'ab'.repeat(3);",
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"expected": "\"ababab\"",
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"gotcha": "Negative n throws RangeError."
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},
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{
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"method": "padStart",
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"tier": "core",
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"task": "Left-pad a string to a length (e.g. zero-pad)",
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"call": "s.padStart(len, '0')",
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"mutates": false,
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"returns": "new string",
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"signature": "s.padStart(targetLength, padString)",
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"example": "return '5'.padStart(3,'0');",
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"expected": "\"005\""
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},
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{
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"method": "padEnd",
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"tier": "core",
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"task": "Right-pad a string to a length",
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"call": "s.padEnd(len, ' ')",
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"mutates": false,
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"returns": "new string",
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"signature": "s.padEnd(targetLength, padString)",
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"example": "return 'ab'.padEnd(5,'.');",
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"expected": "\"ab...\""
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},
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{
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"method": "replace",
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"tier": "core",
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"task": "Replace occurrences of a pattern (first only vs all)",
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"call": "s.replace(pattern, replacement)",
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"mutates": false,
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"returns": "new string",
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"signature": "s.replace(patternOrString, replacementOrFn)",
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"example": "return ['aaa'.replace('a','b'),'aaa'.replace(/a/g,'b')];",
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"expected": "[\"baa\",\"bbb\"]",
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"gotcha": "A string pattern replaces only the FIRST match. Use /x/g or replaceAll."
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},
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{
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"method": "replaceAll",
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"tier": "core",
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"task": "Replace ALL occurrences of a substring",
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"call": "s.replaceAll(sub, replacement)",
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"mutates": false,
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"returns": "new string",
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"signature": "s.replaceAll(patternOrString, replacementOrFn)",
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"example": "return 'aaa'.replaceAll('a','b');",
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"expected": "\"bbb\"",
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"gotcha": "A RegExp argument must have the g flag or it throws."
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},
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{
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"method": "at",
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"tier": "core",
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"task": "Get the LAST character (or nth from the end)",
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"call": "s.at(-1)",
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"mutates": false,
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"returns": "1-char string, or undefined",
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"example": "return ['abc'.at(-1),'abc'.at(0)];",
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"expected": "[\"c\",\"a\"]"
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},
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{
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"method": "match",
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"tier": "core",
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"task": "Find all regex matches in a string",
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"call": "s.match(/\\d+/g)",
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"mutates": false,
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"returns": "array of matches, or null if none",
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"example": "return ['a1b22'.match(/\\d+/g),'abc'.match(/\\d+/g)];",
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"expected": "[[\"1\",\"22\"],null]",
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"gotcha": "Without the g flag you get a single match object, not all matches. null → use ?? []."
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},
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{
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"method": "matchAll",
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"tier": "core",
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"task": "Iterate all regex matches including capture groups",
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"call": "[...s.matchAll(/(\\d)(\\w)/g)]",
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"mutates": false,
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"returns": "iterator of match arrays",
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"example": "return [...'1a2b'.matchAll(/(\\d)(\\w)/g)].map((m)=>m[1]+m[2]);",
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"expected": "[\"1a\",\"2b\"]",
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"gotcha": "The regex must have the g flag or it throws."
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},
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{
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"method": "trimStart",
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"tier": "rare",
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"task": "Strip whitespace from the START only",
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"call": "s.trimStart()",
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"mutates": false,
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"returns": "new string",
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"example": "return ' a '.trimStart();",
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"expected": "\"a \""
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},
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{
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"method": "trimEnd",
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"tier": "rare",
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"task": "Strip whitespace from the END only",
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"call": "s.trimEnd()",
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"mutates": false,
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"returns": "new string",
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"example": "return ' a '.trimEnd();",
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"expected": "\" a\""
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},
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{
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"method": "lastIndexOf",
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"tier": "rare",
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"task": "Find the LAST index of a substring",
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"call": "s.lastIndexOf(sub)",
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"mutates": false,
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"returns": "last index, or -1",
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"example": "return 'hello'.lastIndexOf('l');",
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"expected": "3"
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},
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{
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"method": "substr",
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"tier": "rare",
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"task": "Substring by start index and LENGTH (deprecated)",
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"call": "s.substr(start, length)",
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"mutates": false,
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"returns": "new string",
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"signature": "s.substr(start, length) // deprecated; use slice",
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"example": "return 'abcdef'.substr(1,3);",
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"expected": "\"bcd\"",
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"gotcha": "Second arg is a length, not an end index. Deprecated."
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},
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{
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"method": "concat",
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"tier": "rare",
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"task": "Concatenate strings with a method call",
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"call": "s.concat(a, b)",
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"mutates": false,
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"returns": "new string",
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"example": "return 'a'.concat('b','c');",
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"expected": "\"abc\"",
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"gotcha": "Just use + or a template literal."
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},
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{
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"method": "search",
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"tier": "rare",
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"task": "Find the index of a regex match",
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"call": "s.search(/c/)",
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"mutates": false,
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"returns": "index, or -1",
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"example": "return 'abc'.search(/c/);",
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"expected": "2"
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},
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{
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"method": "localeCompare",
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"tier": "rare",
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"task": "Compare two strings for sorting",
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"call": "a.localeCompare(b)",
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"mutates": false,
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"returns": "negative / 0 / positive number",
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"example": "return ['b','a','c'].sort((x,y)=>x.localeCompare(y));",
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"expected": "[\"a\",\"b\",\"c\"]",
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"gotcha": "Locale-aware and slower; for plain code-unit order use a < b ? -1 : 1."
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},
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{
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"method": "codePointAt",
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"tier": "rare",
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"task": "Get the full Unicode code point at index i",
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"call": "s.codePointAt(i)",
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"mutates": false,
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"returns": "number",
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"example": "return '😀'.codePointAt(0);",
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"expected": "128512",
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"gotcha": "charCodeAt would return only half of the surrogate pair."
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},
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{
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"method": "normalize",
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"tier": "rare",
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"task": "Normalize Unicode (e.g. strip accents with NFD)",
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"call": "s.normalize('NFD')",
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"mutates": false,
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"returns": "new string",
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"example": "return 'é'.normalize('NFD').length;",
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"expected": "2"
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}
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]
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