JavaScript array iterator methods accept a function as their first argument and invoke that function once for each element (or some elements) of the array.
\nIn this post you will come across a term called predicate function, which is defined as a function that either returns true or false.
\nlet nums = [1, 2, 3, 4],\n sum = 0\n\nnums.forEach(num => (sum += num))\n\nconsole.log(sum) // 10let nums = [1, 2, 3, 4],\n\nnums.forEach((v, i, a) => a[i] = v + 1);\n\nconsole.log(nums) // [2, 3, 4, 5]let nums = [1, 2, 3, 4]\n\nconst newNums = nums.map(num => num * num)\n\nconsole.log(newNums) // [1, 4, 9, 16]The callback also takes an optional index (the index of the current element being processed in the array) argument.
\nlet nums = [1, 2, 3, 4]\n\nconst newNums = nums.map((num, index) => num * index)\n\nconsole.log(newNums) // [0, 2, 6, 12]let nums = [1, 2, 3, 4]\n\nconst newNums = nums.filter(num => num < 3)\n\nconsole.log(newNums) // [1, 2]let nums = [1, 2, 3, 4]\n\nconst newNums = nums.every(num => num < 10)\n\nconsole.log(newNums) // truelet nums = [1, 2, 3, 4]\n\nconst newNums = nums.some(num => num > 10)\n\nconsole.log(newNums) // falselet name = ["keith", "lemon", "keith", "lemon"]\n\nconst nameIndex = name.findIndex(name => name === "lemon")\n\nconsole.log(nameIndex) // 1let name = ["keith", "lemon", "keith", "lemon"]\n\nconst nameFound = name.find(name => name.length === 5)\n\nconsole.log(nameFound) // keithlet nums = [1, 2, 3, 4, 5]\n\nconst newNums = nums.reduce((x, y) => x + y)\n\nconsole.log(newNums) // 15let nums = [1, 2, 3, 4, 5]\n// The second argument is 100.\nconst newNums = nums.reduce((x, y) => x + y, 100)\n\nconsole.log(newNums) // 115