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Get the index with the second callback parameter
const items = ["apple", "banana", "cherry"];
items.forEach((item, index) => {
console.log(index, item);
});This logs 0 apple, 1 banana, and 2 cherry. The TypeScript documentation specifies the forEach callback parameters in this order: array element, index, and containing array (TypeScript: Type Compatibility).
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Name the second parameter index or i, whichever makes the code clearest. You can omit it when you only need the item:
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items.forEach((item) => {
console.log(item);
});Use the third parameter when you need the array
The callback’s optional third parameter is the array being traversed. For example, it can be useful when a callback needs to refer to the current array as well as the item and its position:
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items.forEach((item, index, array) => {
console.log(array[index], item);
});Choose a different loop only when it fits the task
Use a counted for loop for explicit index control
If you need to manage the index yourself or control loop flow, use a counted loop:
for (let index = 0; index < items.length; index++) {
console.log(index, items[index]);
}With TypeScript’s noUncheckedIndexedAccess option enabled, an indexed array access may be treated as possibly undefined. The forEach callback instead passes the current element directly. See the TypeScript 4.1 release notes for this option.
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Use for...of when you only need values
A for...of loop gives you each value, but not its index. It is a good fit when position does not matter.
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for...in iterates keys; for an array, those keys are strings. It is intended for inspecting object properties, not as a substitute for the numeric index supplied to a forEach callback. The TypeScript documentation describes these loop behaviors in Iterators and Generators.
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