[
  {
    "method": "push",
    "tier": "core",
    "task": "Append x to the END of arr",
    "call": "arr.push(x)",
    "mutates": true,
    "returns": "the NEW LENGTH (not the array)",
    "example": "const a=[1,2]; const r=a.push(3); return [r,a];",
    "expected": "[3,[1,2,3]]"
  },
  {
    "method": "pop",
    "tier": "core",
    "task": "Remove and get the LAST element of arr",
    "call": "arr.pop()",
    "mutates": true,
    "returns": "the removed element (undefined if empty)",
    "example": "const a=[1,2,3]; const r=a.pop(); return [r,a];",
    "expected": "[3,[1,2]]"
  },
  {
    "method": "unshift",
    "tier": "core",
    "task": "Add x to the START of arr",
    "call": "arr.unshift(x)",
    "mutates": true,
    "returns": "the NEW LENGTH",
    "example": "const a=[2,3]; const r=a.unshift(1); return [r,a];",
    "expected": "[3,[1,2,3]]",
    "gotcha": "O(n): every element shifts. Don't use in hot loops."
  },
  {
    "method": "shift",
    "tier": "core",
    "task": "Remove and get the FIRST element of arr",
    "call": "arr.shift()",
    "mutates": true,
    "returns": "the removed element (undefined if empty)",
    "example": "const a=[1,2,3]; const r=a.shift(); return [r,a];",
    "expected": "[1,[2,3]]",
    "gotcha": "O(n). A BFS queue via shift() is fine on LeetCode sizes, but use an index pointer for big inputs."
  },
  {
    "method": "splice",
    "tier": "core",
    "task": "Remove k elements at index i and/or insert items there, in place",
    "call": "arr.splice(i, k, ...items)",
    "mutates": true,
    "returns": "ARRAY of the removed elements",
    "signature": "arr.splice(start, deleteCount, ...itemsToInsert)",
    "example": "const a=[1,2,3,4]; const r=a.splice(1,2,9); return [r,a];",
    "expected": "[[2,3],[1,9,4]]",
    "gotcha": "Insert only: arr.splice(i, 0, x). Second arg is a COUNT, not an end index."
  },
  {
    "method": "slice",
    "tier": "core",
    "task": "Copy arr[i..j) into a new array",
    "call": "arr.slice(i, j)",
    "mutates": false,
    "returns": "new array (shallow copy); end is exclusive",
    "signature": "arr.slice(start, end)  // end exclusive, negatives count from the end, slice() = shallow copy",
    "example": "return [[1,2,3,4].slice(1,3),[1,2,3,4].slice(-2)];",
    "expected": "[[2,3],[3,4]]"
  },
  {
    "method": "concat",
    "tier": "core",
    "task": "Join arrays into a new array",
    "call": "a.concat(b, c)",
    "mutates": false,
    "returns": "new array",
    "example": "return [1].concat([2],[3,4]);",
    "expected": "[1,2,3,4]",
    "gotcha": "[...a, ...b] does the same."
  },
  {
    "method": "indexOf",
    "tier": "core",
    "task": "Find the index of x in arr (-1 if absent)",
    "call": "arr.indexOf(x)",
    "mutates": false,
    "returns": "first index, or -1",
    "example": "return [[1,2,3].indexOf(2),[1,2,3].indexOf(9)];",
    "expected": "[1,-1]",
    "gotcha": "Uses ===, so it can't find NaN; includes() can."
  },
  {
    "method": "lastIndexOf",
    "tier": "core",
    "task": "Find the LAST index of x in arr",
    "call": "arr.lastIndexOf(x)",
    "mutates": false,
    "returns": "last index, or -1",
    "example": "return [1,2,1].lastIndexOf(1);",
    "expected": "2"
  },
  {
    "method": "includes",
    "tier": "core",
    "task": "Check whether arr contains x",
    "call": "arr.includes(x)",
    "mutates": false,
    "returns": "boolean",
    "example": "return [[1,2,3].includes(2),[NaN].includes(NaN)];",
    "expected": "[true,true]"
  },
  {
    "method": "find",
    "tier": "core",
    "task": "Get the first element matching a predicate",
    "call": "arr.find((x) => x > 2)",
    "mutates": false,
    "returns": "the element, or undefined",
    "example": "return [1,2,3,4].find((x)=>x>2);",
    "expected": "3"
  },
  {
    "method": "findIndex",
    "tier": "core",
    "task": "Get the INDEX of the first element matching a predicate",
    "call": "arr.findIndex((x) => x > 2)",
    "mutates": false,
    "returns": "index, or -1",
    "example": "return [1,2,3,4].findIndex((x)=>x>2);",
    "expected": "2"
  },
  {
    "method": "findLast",
    "tier": "core",
    "task": "Get the LAST element matching a predicate",
    "call": "arr.findLast((x) => x < 3)",
    "mutates": false,
    "returns": "the element, or undefined",
    "example": "return [1,2,3,4].findLast((x)=>x<3);",
    "expected": "2"
  },
  {
    "method": "filter",
    "tier": "core",
    "task": "Keep only elements matching a predicate",
    "call": "arr.filter((x) => x % 2 === 0)",
    "mutates": false,
    "returns": "new array",
    "example": "return [1,2,3,4].filter((x)=>x%2===0);",
    "expected": "[2,4]"
  },
  {
    "method": "map",
    "tier": "core",
    "task": "Transform every element into a new array",
    "call": "arr.map((x, i) => x * 2)",
    "mutates": false,
    "returns": "new array of same length",
    "example": "return [1,2,3].map((x)=>x*2);",
    "expected": "[2,4,6]",
    "gotcha": "['1','2','3'].map(parseInt) is [1,NaN,NaN] — map passes the index as the 2nd arg. Use map(Number)."
  },
  {
    "method": "reduce",
    "tier": "core",
    "task": "Fold an array into one value (e.g. a sum)",
    "call": "arr.reduce((acc, x) => acc + x, 0)",
    "mutates": false,
    "returns": "the final accumulator",
    "signature": "arr.reduce((acc, cur, idx, arr) => newAcc, initialValue)",
    "example": "return [1,2,3].reduce((acc,x)=>acc+x,0);",
    "expected": "6",
    "gotcha": "Without an initial value an empty array throws TypeError."
  },
  {
    "method": "forEach",
    "tier": "core",
    "task": "Run a side effect for every element",
    "call": "arr.forEach((x, i) => { ... })",
    "mutates": false,
    "returns": "undefined (not chainable)",
    "example": "const out=[]; [1,2,3].forEach((x)=>out.push(x*2)); return out;",
    "expected": "[2,4,6]",
    "gotcha": "Can't break/return early. Use for...of when you need that."
  },
  {
    "method": "some",
    "tier": "core",
    "task": "Check whether ANY element matches",
    "call": "arr.some((x) => x > 2)",
    "mutates": false,
    "returns": "boolean (short-circuits)",
    "example": "return [[1,2,3].some((x)=>x>2),[].some((x)=>x>2)];",
    "expected": "[true,false]"
  },
  {
    "method": "every",
    "tier": "core",
    "task": "Check whether ALL elements match",
    "call": "arr.every((x) => x > 0)",
    "mutates": false,
    "returns": "boolean (short-circuits)",
    "example": "return [[1,2,3].every((x)=>x>0),[].every((x)=>x>0)];",
    "expected": "[true,true]",
    "gotcha": "Empty array → true."
  },
  {
    "method": "sort",
    "tier": "core",
    "task": "Sort numbers ascending (in place)",
    "call": "arr.sort((a, b) => a - b)",
    "mutates": true,
    "returns": "the SAME array (sorted)",
    "signature": "arr.sort((a, b) => a - b)  // ascending; b - a = descending",
    "example": "const a=[10,9,1]; const d=[...a].sort(); a.sort((x,y)=>x-y); return [d,a];",
    "expected": "[[1,10,9],[1,9,10]]",
    "gotcha": "Default sort is LEXICOGRAPHIC: [10,9,1].sort() → [1,10,9]."
  },
  {
    "method": "reverse",
    "tier": "core",
    "task": "Reverse an array in place",
    "call": "arr.reverse()",
    "mutates": true,
    "returns": "the SAME array (reversed)",
    "example": "const a=[1,2,3]; const r=a.reverse(); return [r,a,r===a];",
    "expected": "[[3,2,1],[3,2,1],true]",
    "gotcha": "Need a copy? toReversed() or [...arr].reverse()."
  },
  {
    "method": "join",
    "tier": "core",
    "task": "Join array elements into a string",
    "call": "arr.join(sep)",
    "mutates": false,
    "returns": "string",
    "example": "return [1,2,3].join('-');",
    "expected": "\"1-2-3\"",
    "gotcha": "Default separator is a comma, not empty. Use join('') to concatenate."
  },
  {
    "method": "flat",
    "tier": "core",
    "task": "Flatten nested arrays (any depth)",
    "call": "arr.flat(Infinity)",
    "mutates": false,
    "returns": "new array; default depth is 1",
    "example": "return [1,[2,[3]]].flat(Infinity);",
    "expected": "[1,2,3]"
  },
  {
    "method": "flatMap",
    "tier": "core",
    "task": "Map each element to several and flatten one level",
    "call": "arr.flatMap((x) => [x, x * 2])",
    "mutates": false,
    "returns": "new array",
    "example": "return [1,2].flatMap((x)=>[x,x*2]);",
    "expected": "[1,2,2,4]"
  },
  {
    "method": "fill",
    "tier": "core",
    "task": "Create an array of n zeros",
    "call": "new Array(n).fill(0)",
    "mutates": true,
    "returns": "the SAME array (filled)",
    "signature": "arr.fill(value, start, end)",
    "example": "return new Array(3).fill(0);",
    "expected": "[0,0,0]",
    "gotcha": "fill([]) / fill({}) shares ONE reference across all slots. Use Array.from for a 2D grid."
  },
  {
    "method": "Array.from",
    "tier": "core",
    "task": "Create an array of length n from a function of the index",
    "call": "Array.from({ length: n }, (_, i) => i * i)",
    "mutates": false,
    "returns": "new array",
    "signature": "Array.from(iterableOrArrayLike, mapFn)",
    "example": "return Array.from({length:3},(_,i)=>i*i);",
    "expected": "[0,1,4]",
    "gotcha": "Also converts a Set/Map/string: Array.from(set)."
  },
  {
    "method": "Array.from (2D grid)",
    "tier": "core",
    "task": "Create an m×n grid of zeros (rows must be independent)",
    "call": "Array.from({ length: m }, () => Array(n).fill(0))",
    "mutates": false,
    "returns": "new array of independent rows",
    "example": "const g=Array.from({length:2},()=>Array(3).fill(0)); g[0][0]=1; return g;",
    "expected": "[[1,0,0],[0,0,0]]",
    "gotcha": "Array(m).fill(Array(n).fill(0)) makes every row the SAME array."
  },
  {
    "method": "Array.isArray",
    "tier": "core",
    "task": "Check whether a value is an array",
    "call": "Array.isArray(x)",
    "mutates": false,
    "returns": "boolean",
    "example": "return [Array.isArray([]),Array.isArray('a')];",
    "expected": "[true,false]",
    "gotcha": "typeof [] is 'object', so typeof can't tell."
  },
  {
    "method": "at",
    "tier": "core",
    "task": "Get the LAST element (or the nth from the end)",
    "call": "arr.at(-1)",
    "mutates": false,
    "returns": "element, or undefined",
    "example": "return [[1,2,3].at(-1),[1,2,3].at(0)];",
    "expected": "[3,1]",
    "gotcha": "arr[-1] is undefined — only at() takes negatives."
  },
  {
    "method": "entries",
    "tier": "core",
    "task": "Loop with both index and value",
    "call": "for (const [i, x] of arr.entries()) { ... }",
    "mutates": false,
    "returns": "iterator of [index, value] pairs",
    "example": "return [...['a','b'].entries()];",
    "expected": "[[0,\"a\"],[1,\"b\"]]"
  },
  {
    "method": "length",
    "tier": "core",
    "task": "Empty an array in place",
    "call": "arr.length = 0",
    "mutates": true,
    "returns": "(assignment) — array is truncated",
    "example": "const a=[1,2,3]; a.length=0; return a;",
    "expected": "[]",
    "gotcha": "Aliases see the change; arr = [] only rebinds the variable."
  },
  {
    "method": "toSorted",
    "tier": "core",
    "task": "Get a sorted COPY (don't touch the original)",
    "call": "arr.toSorted((a, b) => a - b)",
    "mutates": false,
    "returns": "new sorted array",
    "example": "const a=[3,1,2]; return [a.toSorted((x,y)=>x-y),a];",
    "expected": "[[1,2,3],[3,1,2]]",
    "gotcha": "ES2023; same lexicographic default as sort()."
  },
  {
    "method": "toReversed",
    "tier": "core",
    "task": "Get a reversed COPY (don't touch the original)",
    "call": "arr.toReversed()",
    "mutates": false,
    "returns": "new reversed array",
    "example": "const a=[1,2,3]; return [a.toReversed(),a];",
    "expected": "[[3,2,1],[1,2,3]]"
  },
  {
    "method": "findLastIndex",
    "tier": "rare",
    "task": "Get the INDEX of the last element matching a predicate",
    "call": "arr.findLastIndex((x) => x < 3)",
    "mutates": false,
    "returns": "index, or -1",
    "example": "return [1,2,3,4].findLastIndex((x)=>x<3);",
    "expected": "1"
  },
  {
    "method": "reduceRight",
    "tier": "rare",
    "task": "Fold an array from right to left",
    "call": "arr.reduceRight((acc, x) => acc + x, '')",
    "mutates": false,
    "returns": "the final accumulator",
    "example": "return ['a','b','c'].reduceRight((acc,x)=>acc+x,'');",
    "expected": "\"cba\""
  },
  {
    "method": "copyWithin",
    "tier": "rare",
    "task": "Copy part of the array onto another part of itself, in place",
    "call": "arr.copyWithin(target, start, end)",
    "mutates": true,
    "returns": "the SAME array",
    "signature": "arr.copyWithin(target, start, end)",
    "example": "return [1,2,3,4,5].copyWithin(0,3);",
    "expected": "[4,5,3,4,5]"
  },
  {
    "method": "keys",
    "tier": "rare",
    "task": "Iterate over the indices of an array",
    "call": "arr.keys()",
    "mutates": false,
    "returns": "iterator of indices",
    "example": "return [...['a','b','c'].keys()];",
    "expected": "[0,1,2]"
  },
  {
    "method": "values",
    "tier": "rare",
    "task": "Iterate over the values of an array as an iterator",
    "call": "arr.values()",
    "mutates": false,
    "returns": "iterator of values",
    "example": "return [...['a','b'].values()];",
    "expected": "[\"a\",\"b\"]"
  },
  {
    "method": "toSpliced",
    "tier": "rare",
    "task": "Get a COPY with items removed/inserted (splice without mutation)",
    "call": "arr.toSpliced(i, k, ...items)",
    "mutates": false,
    "returns": "new array",
    "signature": "arr.toSpliced(start, deleteCount, ...items)",
    "example": "const a=[1,2,3,4]; return [a.toSpliced(1,2,9),a];",
    "expected": "[[1,9,4],[1,2,3,4]]"
  },
  {
    "method": "with",
    "tier": "rare",
    "task": "Get a COPY with one index replaced",
    "call": "arr.with(i, v)",
    "mutates": false,
    "returns": "new array",
    "example": "const a=[1,2,3]; return [a.with(1,9),a];",
    "expected": "[[1,9,3],[1,2,3]]"
  },
  {
    "method": "Array.of",
    "tier": "rare",
    "task": "Create an array from the given arguments",
    "call": "Array.of(7)",
    "mutates": false,
    "returns": "new array",
    "example": "return [Array.of(7),Array(3).length];",
    "expected": "[[7],3]",
    "gotcha": "Array(7) makes 7 empty slots; Array.of(7) makes [7]."
  }
]